The method for forming the rough surface of the silicon substrate and the electrolyte for anodic etching of silicon substrates

 

Usage: to create the silicon substrates with surfaces used as emitters ions in analytical instruments, in particular the mass spectrometer. The inventive method of forming a rough surface of a silicon substrate by anodic etching in an electrolyte containing an aqueous solution of hydrofluoric acid and alcohol, etching is carried out in the electrolyte at a current density of not more than 4 mA/cm2optionally containing I2or iodine-containing compound, dissociating the electrolyte with the formation of iodide ions, with subsequent processing of the received optical radiation in the presence of water with intensity, lower damage threshold above the surface. Proposed composition of the electrolyte for anodic etching of silicon substrates comprising an aqueous solution of hydrofluoric acid and alcohol, which further comprises (I2or iodine-containing compound, dissociating the electrolyte with the formation of iodide ions at a certain ratio of components. The technical result of the invention to provide a non-porous or monopolistic silicon substrates with a high degree of surface roughness, obese in the methods of mass spectrometry and ion mobility spectrometry. The invention allows to maintain the stability of the chemical state of the surface of the substrate during prolonged storage under natural conditions. 2 S. and 6 C. p. F.-ly, 9 Il.

The invention relates to a process for electrochemical treatment of semiconductor wafers and can be used to create silicon substrates with surfaces that are applicable as emitters ions in analytical instruments, in particular the mass spectrometer.

The creation of a structurally heterogeneous surface of the silicon substrate is carried out, usually by chemical anisotropic etching. While the etching rate is different for different areas of the silicon surface, depending on the directions of the crystallographic axes of the silicon and places of dislocations and other defects in the crystal structure. As a result of such processing on the silicon surface is formed microstructural roughness.

The possibility of using a silicon substrate, which surface has the microstructure, surface roughness, as a source of ions of different molecules shown in the works of S. Alimpiev, S. Nikiforov, Karavanskii V., Sunner J. On the mechanism of ionization in the method of "SALDI". J. Chem. Phys., 2001, V. 115, p 1891-1901; Zhouxin Shen, John J. Thomas, Claudia Averbuj, Klas is a process for the formation of ions includes a number of serial and parallel stages, including, in particular, the adsorption of defined connections (in case of finding it in the gas phase), the dissociation on the surface ionization on the surface due to the proton transfer or separation of the electron and desorption of the formed ions from the surface in the laser or heat. The effectiveness of the process of formation of ions, i.e. the ratio of the number of charged particles to the number of neutral particles, debarbieux per unit time per unit surface is largely determined by the degree of roughness (in particular, chemically polished surface of the silicon ionization degree is close to zero). This is due to several reasons, in particular: - the effect of amplification of the magnitude of the electric field at the edges (the bumps) of a rough surface. Due to the small size of these bumps, the electric field can reach large values, providing efficient charge separation at the surface; increasing the specific surface value, which leads to an increase in the number of sorbed molecules per unit time; the increase in the degree of dispersion and, consequently, the catalytic activity of the surface of a silicon substrate, which is the driving force for the sorption and ionization of molecules, can be used a substrate made of porous silicon. These substrates, along with the system going into the volume of the narrow and deep then, have a rough surface. The porosity of the surface is not a necessary condition for producing ions, while microstructural roughness necessary.

A method of processing a silicon substrate for formation of a layer of porous silicon on the surface of the substrate, including anodic etching of n-si in the range of current density from 30 to 150 mA/cm2during the time from 20 to 600 minutes in concentrated hydrofluoric acid under illumination of the working surface of silicon, for example, an incandescent lamp with a capacity of 200-500 W [RF Patent 1459542, CL H 01 L 21/306, publ. 2000].

The known method has the following disadvantages.

The obtained coarse surface structure leads to low efficiency of ionization of molecules on these surfaces.

Furthermore, the porous layer has a large thickness. This layer has a high sorption capacity and selectivity adsorb from the gas phase molecules defined and interfering components. For quite a long time adsorbed is cepelinai the impossibility to implement the analysis of the gas phase in real time using porous silicon surface.

There is a method of electrochemical machining of semiconductor wafers, including the formation of porous silicon on the surface of silicon substrates (for example, KDB-12(001) in the tub with a water solution of hydrofluoric acid at a current density of 20 mA/cm2[RF patent 2133997, CL H 01 L 21/3063, publ. 1999]. The uniformity of the layer of porous silicon controlled by the refractive index at a wavelength of 0.63 µm laser digital ellipsometer LST-601. The refractive index by 10-13 points on the surface of the wafer was 2.2+/-0,3 when the dispersion value of 0.7.

The obtained coarse surface structure leads to low efficiency of ionization of molecules on these surfaces.

Furthermore, the porous layer has a large thickness. This layer has a high sorption capacity and selectivity adsorb from the gas phase molecules defined and interfering components. For quite a long time adsorbed in the pores of the molecules diffuse from volume to work (rough) surface, "poisoning" her. This leads to the fundamental impossibility to implement the analysis of the gas phase in real time using porous silicon.

Isvee water [RF Patent 2133997, CL H 01 L 21/3063, publ. 1999] and the composition of the electrolyte for anodic etching of silicon substrates, consisting of 10-48% solution of hydrofluoric acid [RF Patent 2065640, CL H 01 L 21/304, publ. 1996].

Known compositions of the electrolytes do not allow to obtain microstructural roughness of the surface of silicon substrates, which leads to low efficiency of ionization of molecules on these surfaces. And a porous layer obtained by etching the known electrolytes, it is not possible to implement the analysis of the gas phase in real time due to diffusion from the pores to the surface defined and interfering components.

The closest technical solution is the method of processing a silicon substrate by anodic etching in an electrolyte containing an aqueous solution of hydrofluoric acid and ethanol [J. J. Thomas et al., Desorption-ionization on silicon mass spectrometry: an application in forensics, J. Analytica Chimica acta 442, 2001, p.185].

In the known method, the silicon substrate is placed in a Teflon cell containing 25% aqueous solution of hydrofluoric acid and ethanol, and conduct anodic etching at a current density of 5 mA/cm2within 1 minute. Next, the substrate is treated with ozone followed by etching in a mixture, the content is.

The surface of the substrate as a result of said processing sufficiently obtained rough and porous (thickness less than 1 μm), which leads to its use as an emitter of ions in analytical devices.

However, the known method has a number of disadvantages.

Received a relatively small degree of surface roughness of porous silicon, which leads to relatively low efficiency of ionization of molecules on these surfaces.

In addition, when processing a silicon substrate by a known method get though to a lesser extent than in counterparts, but still quite thick porous layer. This layer has a high sorption capacity and selectivity adsorb from the gas phase molecules defined and interfering components. For quite a long time adsorbed in the pores of the molecules diffuse from volume to work (rough) surface, "poisoning" her. This leads to the fundamental impossibility to implement the analysis of the gas phase in real time using the surface of porous silicon obtained by a known method.

An additional disadvantage of this method of treatment is by the inability to store the substrate in aerobic conditions. Upon contact with the air is sufficiently rapid oxidation of the surface silicon atoms. Formed as a result of oxidation of the oxide layer is an insulator, which excludes the possibility of applying such a surface as the emitter of ions in analytical devices. Therefore, when using silicon substrates prepared by a known method, an ion emitter you can either use only freshly prepared samples, or during storage to take special measures to prevent contact of the substrate with air.

The closest composition to the invention is an electrolyte for anodic etching of silicon substrates comprising an aqueous solution of hydrofluoric acid and ethanol [J. J. Thomas et al., Desorption-ionization on silicon mass spectrometry: an application in forensics, J. Analytica Chimica acta 442, 2001, p.185].

Known electrolyte composition contains 25% aqueous solution of hydrofluoric acid and ethanol.

When using the above electrolyte, the surface of the substrate sufficiently obtained rough and with a smaller thickness of the porous layer, which leads to its use as an emitter of ions in analytical devices.

However, the known composition of the electrolyte obligatorily silicon substrates with a high degree of surface roughness, providing higher in comparison with the prototype, the efficiency of ionization of the molecules and, accordingly, the higher the sensitivity analysis in the methods of mass spectrometry and ion mobility spectrometry. In addition, the invention allows to maintain the stability of the chemical state of the surface of the substrate during prolonged storage under natural conditions.

The technical problem is solved in that in the method of forming a rough surface of a silicon substrate by anodic etching in an electrolyte containing an aqueous solution of hydrofluoric acid and alcohol, etching is carried out in the electrolyte at a current density of not more than 4 mA/cm2optionally containing I2or iodine-containing compound, dissociating the electrolyte with the formation of iodide ions, with subsequent processing of the received optical radiation in the presence of water with intensity, lower damage threshold above the surface.

It is advisable as iodine-containing compounds to use ammonium iodide, potassium iodide, sodium iodide.

Preferably anodic etching to maintain no more than 10 minutes.

Appropriate wavelength of optical radiation to choose from the condition of absorption is as a source of optical radiation to use a laser or a broadband source of optical radiation, in particular mercury vapor lamp.

Preferably the processing of optical radiation carried out in the presence of water vapor or in the aquatic environment.

In addition, the problem is solved in that the composition of the electrolyte for anodic etching of silicon substrates comprising an aqueous solution of hydrofluoric acid and alcohol, it also contains the I2or iodine-containing compound, dissociating the electrolyte with the formation of iodide ions, in the following ratio of components,% vol.: HF in the form of a 50% aqueous solution of 20-50 I2or iodine-containing compound in the form of a 5% solution in alcohol - 10-40 Alcohol - Rest Fig. 1. View of the surface of a silicon substrate manufactured by the method of the prototype.

Fig.2. View of the surface of a silicon substrate manufactured according to the claimed method described in example 1.

Fig.3. View of the surface of a silicon substrate manufactured according to the claimed method described in example 2.

Fig. 4. The IR spectrum of the surface of a silicon substrate obtained 10 days after manufacturing the substrate according to the method of the prototype.

Fig. 5. The IR spectrum of the surface of a silicon substrate obtained 10 days after manufacturing the substrate for the proposed method (example 2).

Fig. 6. View poverhnosti after etching in the inventive electrolyte and further processing by laser radiation in the presence of water vapor (example 2).

The images shown in Fig.1-3 and 6-7, obtained by scanning atomic force microscope.

Fig. 8. The intensity of the ion signal from the flow of laser radiation when using a substrate obtained by the method of the prototype (1) and proposed method (2).

Fig. 9. The mass spectrum of TNT obtained with a rough surface produced by the present method. The vapor pressure of TNT 10-9Torr.

Anodic etching of the silicon substrate is carried out in a stationary cell etching, representing a two-chamber design. Between cameras gasket is clamped silicon substrate is subjected to etching. As use rubber pads or grease is insoluble in the electrolyte. In the first chamber is placed in a platinum anode output is connected to the positive terminal of the power source. The material of the anode is selected from the condition of insolubility of the anode electrode. In particular, can be used graphite or lead. The first chamber is equipped with connections for input and output of the electrolyte.

The second camera is fitted with a transparent window made of LiF, MgF2or other transparent materials that are resistant to Vostochniy power, and connections for input and output of the electrolyte.

In the first and the second chamber is filled with electrolyte to provide ohmic contact (first camera) and the actual etching process (second camera).

The method is as follows.

For the preparation of the electrolyte take 2-4 ml of alcohol, for example ethanol, poured in a Teflon container 10 ml, add using micropipette 3-4 ml of a 50% aqueous solution of hydrofluoric acid and the same quantity of 5% solution of iodine in ethanol. After stirring for 5-10 minutes ready electrolyte is poured into the second cell.

The same procedure is repeated to fill the first chamber.

After pouring the electrolyte, the electrodes are connected to a power source, which is stable constant current source 1-100 mA. The optimum current density is chosen in the interval up to 4 mA/cm2. After a specified period of time (from several seconds to 10 minutes) the cell is disconnected from the power source. The substrate was washed with distilled water and dried in a stream of nitrogen.

Further processing is done in two ways, leading to immediate results.

1. The prepared substrate is placed in a vacuum Cham is practical radiation, as the source which uses a laser or lamp. As a laser source is used, for example, ND: YAG, working on the second, third or fourth harmonic, or a nitrogen laser. The energy density of the radiation on the surface of the substrate should not exceed the damage threshold of the substrate and using a nitrogen laser is 10-50 MJ/cm2.

2. The prepared substrate is placed in the water bath. The etched surface of the substrate is exposed to optical radiation source which uses a laser or lamp. As a laser source is used, for example ND:YAG, working on the second, third or fourth harmonic, or a nitrogen laser. The energy density of the radiation on the surface of the substrate should not exceed the damage threshold of the substrate and using a nitrogen laser is 10-50 MJ/cm2.

Example 1 in 10 ml of the electrolyte composition: 3 ml of 50% aqueous HF solution, 2.5 ml 5% R-RA in ethanol I2and 4.5 ml of ethanol is placed in a chamber of the cell and carry out the etching of the silicon substrate by the above method at a current density of 1 mA/cm2within 1 min 50 seconds.

Potraviny substrate is washed with water, dried with nitrogen and placed in vaquera (wavelength 337 nm) with energy density on the surface of the substrate 37 MJ/cm2.

Receive the substrate, the surface of which is shown in Fig. Example 2
10 ml of the electrolyte composition: 5 ml of 50% aqueous solution of HF, 1 ml of 5% R-RA in ethanol KI and 4 ml of ethanol are placed in a chamber of the cell and carry out the etching of the silicon substrate by the above method at the current density of 1.5 mA/cm2for 1 min 30 seconds.

Potraviny substrate is washed with water, dried with nitrogen and placed in a water bath. Next, the surface is treated with radiation of a mercury lamp with a power output of 12 Watts.

Receive the substrate, the surface of which is shown in Fig.3.

For detailed mapping of the main physico-chemical parameters characterizing the state of the surface of a silicon substrate obtained by the method of the prototype and the claimed method were analyzed by different methods. The results obtained indicate that the substrate obtained by the claimed method differs from substrates prototype on a number of parameters.

1. The etching process in the electrolyte additionally contains I2or iodine-containing compound, dissociating the electrolyte with the formation of iodide ions at a current density of not more than 4 mA/cm2leads to changes in physical parameters of the surface. In the ulttam, obtained using atomic force microscope (Fig.1, figs.2 and Fig.3), the characteristic dimensions of the roughness of the silicon substrates in the plane of the surface are: 200-500 nanometers from the substrate obtained by the method of the prototype; 50-80 nm at the substrate obtained by the claimed method.

2. The etching process in the electrolyte additionally contains I2or iodine-containing compound, dissociating the electrolyte with the formation of iodide ions at a current density of not more than 4 mA/cm2leads to changes in the chemical properties of the surface. This is illustrated by infrared absorption spectra, shown in Fig. 4 and Fig.5. Shows absorption spectra show that after 10 days of storage in air, the surface of the substrate obtained by the method of the prototype, oxidized, as evidenced by the line Si-OH (peak 1 in Fig.4) and Si-O-Si (peak 2 in Fig.4). At the same time on the IR spectrum of the surface of a silicon substrate obtained by the present method, the above line is largely suppressed (Fig. 5). In addition, the use of the proposed method suppresses the formation of groups Si-Hx(in contrast to Fig.4, peak 3, corresponding to the triplet Si-H, Si-H2Si-H3, Fig.5 missing).

Processing Opti is giving results of the surface analysis by electron microscopy (Fig. 6 and Fig.7) argues further increase the degree of roughness of the substrate. The main microstructural changes occur not in the plane of the surface, and mainly in the direction normal to the surface of a silicon substrate.

To compare the efficiency of ionization of organic molecules on the surface of the silicon substrates obtained dependence of the yield of various ions on the intensity of the laser radiation. In Fig.8 for example, trinitrotoluene presents such dependences are obtained by using a substrate prepared according to the method of the prototype (curve 1) and the substrate prepared by the present method (curve 2). From the data obtained it follows that the use of the claimed method increases the yield of ions under the same experimental conditions, up to 70 times.

Full mass spectrum of TNT obtained using silicon substrates prepared by the present method, shown in Fig.9.


Claims

1. The method of forming a rough surface of a silicon substrate by anodic etching in an electrolyte containing an aqueous solution of hydrofluoric acid and alcohol, characterized in that the etching weewee connection, dissociating the electrolyte with the formation of iodide ions, with subsequent processing of the received optical radiation in the presence of water with intensity, lower damage threshold above the surface.

2. The method of forming a rough surface on p. 1, characterized in that as the iodine-containing compounds using ammonium iodide, or potassium iodide, or sodium iodide.

3. The method of forming a rough surface on p. 1, characterized in that the anodic etching is conducted not more than 10 minutes

4. The method of forming a rough surface on p. 1, characterized in that the wavelength of optical radiation is chosen from the condition of its absorption in the silicon substrate or in a layer of water on the surface of a silicon substrate.

5. The method of forming a rough surface on p. 1, wherein the source of optical radiation using a laser or a broadband source of optical radiation, in particular, mercury vapor lamp.

6. The method of forming a rough surface on p. 1, characterized in that the processing of optical radiation is carried out in the presence of water vapor.

7. The method of forming a rough surface on p. 1, characterized in that the treatment of the substrate, comprising an aqueous solution of hydrofluoric acid and alcohol, characterized in that it additionally contains I2or iodine-containing compound, dissociating the electrolyte with the formation of iodide ions, in the following ratio of components,%:

HF in the form of a 50% aqueous solution of 20-50

I2or iodine-containing compound in the form of a 5% solution in alcohol 10-40

Alcohol The Rest

 

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