Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/67—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere
  • H01L21/67005—Apparatus not specifically provided for elsewhere
  • H01L21/67011—Apparatus for manufacture or treatment
  • H01L21/67017—Apparatus for fluid treatment
  • H01L21/67028—Apparatus for fluid treatment for cleaning followed by drying, rinsing, stripping, blasting or the like
  • H01L21/6704—Apparatus for fluid treatment for cleaning followed by drying, rinsing, stripping, blasting or the like for wet cleaning or washing
  • H01L21/67057—Apparatus for fluid treatment for cleaning followed by drying, rinsing, stripping, blasting or the like for wet cleaning or washing with the semiconductor substrates being dipped in baths or vessels
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
  • C09K13/00—Etching, surface-brightening or pickling compositions
  • C09K13/04—Etching, surface-brightening or pickling compositions containing an inorganic acid
  • C09K13/08—Etching, surface-brightening or pickling compositions containing an inorganic acid containing a fluorine compound
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F1/00—Electrolytic cleaning, degreasing, pickling or descaling
  • C25F1/02—Pickling; Descaling
  • C25F1/04—Pickling; Descaling in solution
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F3/00—Electrolytic etching or polishing
  • C25F3/02—Etching
  • C25F3/12—Etching of semiconducting materials
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/02041—Cleaning
  • H01L21/02043—Cleaning before device manufacture, i.e. Begin-Of-Line process
  • H01L21/02052—Wet cleaning only
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/67—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere
  • H01L21/67005—Apparatus not specifically provided for elsewhere
  • H01L21/67011—Apparatus for manufacture or treatment
  • H01L21/67017—Apparatus for fluid treatment
  • H01L21/67063—Apparatus for fluid treatment for etching
  • H01L21/67075—Apparatus for fluid treatment for etching for wet etching
  • H01L21/67086—Apparatus for fluid treatment for etching for wet etching with the semiconductor substrates being dipped in baths or vessels
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
  • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/547—Monocrystalline silicon PV cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• the present invention relates to a method for electrochemically cleaning and etching solid surfaces simultaneously, in particular the surface of silicon workpieces, e.g. silicon wafers for use in the solar and microelectronics industry, comprising (1 ) the placement of a solid object into an electrochemical chamber comprising (a) a cathode, (b) an anode conductively connected to said object, and (c) a conductive fluid, wherein the fluid or components thereof can be decomposed electrochemically to provide hydrogen gas and/or other gaseous products at a potential below the electrochemical potential of said object, (2) applying a current resulting in a voltage being (i) higher than the decomposition voltage of said fluid or components thereof to form hydrogen gas and/or other gaseous bubbles at the surface of the object, and (ii) preferably lower than the
• multi-wire sawing is the main slicing technique in the photovoltaic and microelectronics industry. Due to its high throughput, small kerf loss, little restriction on the size of ingots as well as its excellent wafer surface quality, this technology dominates over other techniques.
• an inner diameter saw e.g. ID saw TS207 from Meyer-Burger AG, Stefflsburg, Switzerland
• squaring e.g. by band saws with diamond-plated saw bands (e.g.
• the bricked silicon ingot is typically glued on one side to a glass support plate, which is in turn glued to a carrier mount for positioning the brick appropriately within the multi-wire saw.
• This multi-wire cutting device then slices the brick into wafers with a thickness of around 70 to 280 ⁇ , e.g. on a DS 271 multi- wire saw from Meyer-Burger AG, CH.
• the multi-wire cutting process is regularly either a lapping process using abrasive fluid (slurry) or a fixed diamond/sapphire wire cutting process.
• Silicon and sapphire are only a few examples of the typical materials like GaAs, SiC and other alloys that form commercial semiconductor material.
• the hundreds of thin wafers are still attached to the partially cut glass support plate (now having a comb-like structure) on the carrier mount. All wafer surfaces in contact with the wire now either have abrasive slurry (lapping process) or cooling fluid (diamond/sapphire wire cutting process, particularly used for cutting metals) together with silicon dust sticking to them that needs to be completely removed before further processing for photovoltaic or microelectronics purposes. These loose but sticky surface contaminants are typically removed by multiple nozzles guide water jets and treatment with aqueous and organic solvents and
• US 6,418,942 discloses a closed solvent processing system including a vacuum chamber and disposed therein an object and a solvent. By alternating pressure and vacuum within said vacuum chamber decompression bubbles are formed at the objects surface, said bubbles mechanically interacting with said object by generating energy from implosion of said decompression bubbles. The bubble formation and implosion is utilized to clean or dislodge micron and sub-micron particles and insoluble contaminants at the object ' s surface.
• WO 2009/061691 A1 teaches an open aqueous cleaning system including a vessel and disposed therein an object, water and a chemical reactant, preferably an oxidizing agent such as peroxides, ozone, acids, hydroxides, etc. that either reacts with the solid surface or contaminants on the surface.
• a continuous stream of bubbles is produced either by heating the water and/or pulling a vacuum on said vessel and the chemical agent in the vapour bubbles contacting the object ' s surface and surface residues cleans the object chemically and the bubbles remove the reaction residue.
• US 2002/0023663 describes an apparatus and a corresponding method for preventing the re-adherence of particles in a wafer-cleaning process including high pressure gas in a water/cleaning solution. For cleaning the gas bubbles are injected with high power towards the wafer surface. However, this process is not suitable for cleaning semiconductor wafers because of the pressure required.
• US 7,165,563 teaches a method and corresponding apparatus for decoupling power and cavitation for megasonic cleaning applications. Most of the bubbles are generated randomly in the solution, where the bubbles have no effect.
• US 6,488,037 discloses a programmable physical action process for an integrated circuit wafer cleaning method including ultrasonic and bubble treatment with inert gas pressed into the solution enhanced by chemical bathes.
• solid surfaces need to be surface treated, e.g. by mechanical, chemical and electrochemical means.
• the clean or at least pre-cleaned wafers are regularly surface treated, e.g. texturized by electrochemical etching (ECE).
• ECE electrochemical etching
• Etching occurs when an anodic potential is applied that is higher than the electrochemical potential of the reaction that has to be triggered: If the potential is higher than the electrochemical potential of Cu, the Cu impurities in the silicon will be removed, i.e. oxidized.
• Si it is beneficial to remove impurities with a potential lower than the electrochemical potential of Si below the electrochemical potential of Si in order not to etch the silicon itself.
• the impurity concentration in the solution must be lower than in the silicon. This can be achieved by removing impurities from the etching solution. This is typically done by the cathode catching all cationic impurities (especially Cu, Fe, Ni and Co cations) due to its electrical field.
• the cathode can be cleaned by applying a reverse voltage in combination with a temporary cathode, possibly also of high grade Silicon.
• Varying the voltage and measuring the current will reveal what redox reactions at the surface of the silicon take place, revealing the amount of contaminants still present. If metal parts with a protective layer are used in the system (e.g. a steel work piece holder), and this protective layer gets damaged, this can be detected by varying the potential because at the electrochemical potential of iron, the current will jump up.
• a protective layer e.g. a steel work piece holder
• electrochemically formed gas bubbles on solid objects conductively connected to an anode such as e.g. a silicon workpiece, for example a silicon wafer undergoing electrochemical texturizing, can be used for efficiently cleaning the surface of said object when the bubble size is increased and/or the bubble is collapsed in the close vicinity of the surface.
• the bubbles of the inventive method are produced at the location needed, directly at the solid ' s surface that needs to be texturized and cleaned; and consequently the bubbles do not have to diffuse to the solid ' s surface, nor do they have to travel through the narrow gap between neighbouring wafers in wafer arrays.
• the amount of bubbles needed can be controlled by controlling the electric current.
• the gas bubbles expanding due to decompression can better brake away contaminant residues. The same holds true for collapsing bubbles generated by rapid pressure increase.
• Another surprising advantage of this method is that cationic contaminants will diffuse and attach to the cathode, thus being conveniently removed from the fluid system.
• the inventive method for electrochemically cleaning and etching solid surfaces takes advantage of the electrochemically produced gas bubbles that are generated when the conductive fluid or components thereof in contact with the anodic solid object and the cathode is decomposed at the anodic surface of the object.
• the method requires placing the solid object into an electrochemical chamber that utilizes the object as anode and includes a separate cathode.
• the conductive fluid is water or contains water hydrogen gas is produced at the anode.
• any conductive fluid or components thereof that have an electrochemical potential below the electrochemical potential of said anodic object and produce hydrogen gas and/or other gaseous products can be used for the method of the invention.
• the electrochemi- cally generated gas bubbles mechanically impact on the surface, thereby removing adhesive contaminants. These are mechanically broken away by the growing/imploding bubbles.
• the solid object may be any solid object that can function as anode and which has a higher electrochemical potential than the conductive fluid. Thus, the solid object essentially stays intact whereas contaminants with a lower electrochemical potential are electrochemically etched away.
• the solid object to be simultaneously etched and cleaned is selected from the group consisting of metals, metalloids and alloys comprising these, preferably metalloids and alloys selected from the group consisting of silicon, germanium, arsenic, antimony, tellurium, selenium and alloys of two or more of these.
• Typical cationic contaminants in these metals, metalloids and alloys thereof are Au, In, Ga, Ta, Mo, Nb, Zr, W, Ti, V, Cr, Mn, Fe, Co, Al, Ni, Cu, Ca, Cs, Na, Li, Rb, C and typical anionic contaminants are O, CI, F, N, C.
• the solid object to be etched and cleaned is a wafer, preferably a silicon wafer, more preferably an array of silicon wafers.
• Such wafer arrays can be multi-wire cut wafers still arranged in parallel and still glued to, for example a glass support, which can optionally still be attached to a support beam for easier handling during the multi-wire cutting and the subsequent cleaning and processing steps.
• Such a fixed wafer array will allow for processing many, up to hundreds of wafers according to the invention in parallel.
• brush-like connectors or sponge-like connectors e.g. made of carbon, can be employed which will contact and bridge all of the wafer at the same time.
• the wafers of the array may be fixed in place, e.g. like a record collection, and conductively connected by a grid cage or basket.
• a solid silicon object preferably a silicon wafer, more preferably an array of silicon wafers into an electrochemical chamber comprising (i) a cathode, preferably a cathode of pure p- or n-doped silicon, (ii) an anode conductively connected to said silicon object, and (iii) a conductive fluid, wherein the fluid or components thereof can be decomposed electrochemically to provide hydrogen gas and/or other gaseous products at a potential below the electrochemical potential of silicon,
• the solid object is a semiconductor object, preferably one made from silicon or GaAs, and during the method the object is irradiated by light to reduce or prevent passivation.
• the range of the wavelength would be 100 to 1300 nm.
• the wavelength range will correspond or might differ.
• step (3) of changing the pressure of the solution comprises alternating vacuum and pressure within the electrochemical chamber to cause decompression growth followed by pressure-induced collapse of bubbles.
• the range of pressure changes of the fluid i.e. the pressure changes affecting the fluid and the bubble size of the electrochemically generated gases, is selected for aqueous fluid systems at about 50 to 98 °C between 250 and 500 mbar, preferably at about 70 °C between 325 and 375 mbar and more preferably at about 80 °C between 375 and 425 mbar.
• the compression pressure is above and the decompression pressure is below atmospheric pressure.
• the range of the pressure change can stay above or below atmospheric pressure.
• the current and voltage applied to the electrochemical cell are chosen so that the surface of the object is texturized and at the same time gas bubbles are generated electrochemically from the conductive fluid.
• aqueous conductive fluids such as e.g. a fluid comprising H 2 0, HF and HN0 3
• the current would be adapted to provide a voltage of about 0.5 to 1 V, which will produce hydrogen gas bubbles.
• the current depends on the active wafer surface area and the type and shape of the cathode.
• the voltage applied to the conductive fluid is preferably selected from a direct current (DC), a pulsed direct current and a direct current with small reverse pulses.
• DC direct current
• the effect and advantage of a pulsed direct current is to reduce the diffusion zone in the solution and to reduce the diffusion dead layer direct at the anode.
• the effect and advantage of a direct current with small reverses is that the diffusion zone in the electrolyte is mixed which enables to drive higher currents.
• the method of the invention will remove contaminants from the object ' s surface, either by electrochemically reacting with them, removing them mechanically through bubble size action, dissolving them in the conductive fluid, reacting them with reactive components in the conductive fluid, e.g. HF, HN0 3 , alkaline hydroxides, etc. or by electrostatically diffusing cationic contaminants to the cathode.
• reactive components in the conductive fluid e.g. HF, HN0 3 , alkaline hydroxides, etc. or by electrostatically diffusing cationic contaminants to the cathode.
• the inventive method includes a further step (4) for the removal of contaminants from the fluid, preferably by skimming, settling, coagulating, filtering, changing the fluid.
• the steps of the inventive method include a further step (5) for the removal of cationic contaminants from the cathode.
• One quick way is by simply replacing a dirty for a clean cathode.
• the rising bubbles will encapsule or drag on their surface with them contaminants. Therefore, a filter above the rising bubbles can conveniently capture and immobilize and/or remove contaminants. Therefore, the method of the invention preferably includes a further step (4a) for the removal of contaminants from the fluid, wherein a filter is positioned above the solid object, so that rising contaminated bubbles transport the contaminants to the filter.
• the hydrogen gas and/or other gaseous bubbles generated electrochemically in the methods of the invention that are removed for pressure reduction purposes are preferably recycled at least partly as a pressurizing medium or part thereof.
• the conductive fluid and/or the solid object are heated to expand bubble volume, thereby further enhancing the mechanical effects of the bubbles and increasing the solvent effect of the conductive fluid.
• the conductive fluid can be an aqueous or a non-aqueous conductive fluid. Only the above requirements regarding its electrochemical and decomposition potential to form gas bubbles are mandatory.
• HF HF
• an oxidizing agent preferably H 2 0 2 or HN0 3
• an organic water-miscible solvent preferably an alcohol, more preferably methanol, ethanol, propanol and/or isopropanol for acidic fluids
• the conductive fluid is a non-aqueous conductive fluid selected from the group consisting of (i) HF, HAIF 3 , HBF 4 , HPF 6 , HCF3SO3, HAsF 6 and/or HSbF 6 plus oxidizing agent, preferably HN0 3 , and an organic solvent, preferably an alcohol, preferably methanol, ethanol, propanol and/or isopropanol, and/or acetonitrile and/or dimethylformamide.
• the contaminant concentration in the electrochemi- cally treated object directly affects the electrochemical current, thus providing for a convenient indicator of contaminant removal.
• the method of the invention is one, wherein in the electrochemical current, preferably at a specific potential, is monitored to determine the type of impurity and the amount of contaminants present on the object ' s surface.
• the conductive fluid is chosen such that the electrochemical potential of the contaminant to be removed is lower than the electrochemical potential of the object treated.
• the conductive fluid can be designed to suit one or more specific contaminants.
• several different conductive fluids with different compositions and/or different electrochemical potentials can be prepared and utilized in the inventive method consecutively.
• more than one conductive fluid is used consecutively to remove different contaminants.
• the electrodes can generate a so-called “dead” fluid layer directly attached to their surface, the movement of which is not controlled by free diffusion but which is “attached” by Van der Waals and electrostatic forces of the solved molecules. All physical and chemical interactions are affected by this immobilized layer which can be measured as a so called electrochemical survoltage. Due to these physical attachment effects of the fluid the concentration of molecules and impurities in the fluid near the solid to be treated and functioning as an electrode cannot be controlled. When removing the wafers after electrochemical cleaning and etching, there is a high risk to carry over almost the complete or at least part of the immobilized layer together with the wafer(s).
• the anionic contaminants can be affected in the same way.
• the cationic ions are associated with negatively charged substances, e.g. in water the electronegatively charged hydroxide ions will be drawn towards cations.
• the association of cationic and anionic substances in the fluid will increase their effective mass and hydrodynamic radius and will slow down their acceleration in the electrical field. Therefore, a short time polarity reversal of the electrical field will not push the previously removed cations back to the wafer(s).
• Anions typically have a smaller hydrodynamic volume resulting in less electrostatic effect compared to similarly charged cations.
• Anions are typically slower than cations in an electrical field. Therefore, a short time polarity reversal is generally sufficient for the ions to escape the "dead” zone. Both effects, i.e. the breaking up of the "dead” layer by bubble generation, expansion and/or implosion and the repelling of the cations and anions from the "dead" fluid layer can be combined in both phases of voltage reversal. If the mobilities of the ionic contaminants in the fluid are known the pulse duty factor can be optimized.
• inventive method relates to a method for electrochemically cleaning and etching solid surfaces comprising the following steps:
• step (2 ' ) the voltage applied in step (2 ' ) is reversed at least once for a time period sufficient to brake up a "dead" fluid layer immobilized on the solid ' s surface.
• a preferred example is an array of sliced wafers, more preferably silicon wafer, arranged in a workpiece holder like a grid cage or still glued to the glass support from a previous multi-wire cutting process of a silicon brick.
• every second solid object, e.g. wafer functions as anode and every first solid object functions as cathode, thus alternating anode and cathode.
• the anode/cathode arrangement could also start with an anode.
• the reversal of the voltage applied at least once should preferably be for a time period sufficient to brake up a "dead" fluid layer immobilized on the solid ' s surface but short enough to avoid recontamination of the treated anodic object surfaces.
• the suitable time range for voltage reversal will depend on the objects, the distance between objects, the conductivity of the fluid and amongst other factors the applied reverse voltage.
• step (3) is performed
• an array or fixed stack of objects e.g. silicon wafers can be alter- natingly connected to a different potential by means of a conductive glue.
• the thickness of the glue is preferably less than the gap between the wafers.
• a conductive glue with anisotropic conductivity (the conductivity of the beam to the wafer should be higher than the conductivity from wafer to wafer) can be used and a beam consisting of a printed circuit board with electrodes in a grid corresponding the wafer arrangement can be used.
• the alternatingly charged solid object arrangement comprises a separately located cathode located outside the solid object arrangement, preferably having the most negative potential to take up all the cationic impurities in the fluid medium, thereby having a cleaning effect on fluid contaminants.
• the electric field from the further cathode will not have an effect too deep into the wafer gap distances.
• a periodic reversing of the electrical field is preferred. It is also preferred to apply a constant voltage to every second object, leading to enhanced cleaning of every second anodic object.
• the further cathode is no longer necessary in the multi-anode-multi- cathode setting, but can still be used as a most negative cathode and drain for cationic contaminants.
• the present invention relates to a method of the invention, wherein subsequently to electrochemically cleaning and etching the solid surfaces the solid surfaces are electrochemically or chemically oxidized to generate a protective oxide layer. Conveniently, this can be done in the same electrochemical chamber used for the previous steps.
• electrochemical chamber walls, workpiece holders and any other hardware coming into contact with the conductive fluid should be essentially metal free and non- conductive, e.g. plastic, glass, ceramics, etc.
• steps (1) and (2) as well as steps (1 ' ) and (2 ' ) can be combined with steps (3) and (3 ' ), respectively.
• Fig. 1 is a schematic drawing of a pressure controlled electrochemical chamber (1) including a silicon wafer array (2) still glued to glass support (2a) attached to a support beam (2b), each of the wafers being conductively connected to an anode (3) by a carbon brush (4) and each wafer forming part of the anode (3), a cathode (5), a pressure/vacuum tube (6) and a current generator (7). Further features include electrochemically generated gas bubbles (8), released contaminants (9), gas volume above the conductive fluid (10), conductive fluid volume (11).
• a silicon wafer array (2) of about 350 wafers having a 15,6 cm square plane size, a width of about 180 pm and a wire-cut distance between them of about 150 nm, that was previously produced from a squared silicon ingot by multi-wire sawing and already pre-cleaned by the jet action of water nozzles and ultrasound treatment in water tanks followed by multiple washes with aqueous solvents is left glued to the partially cut glass plate (2a) attached to a support beam (2b).
• the complete silicon wafer array (2) is submersed into the conductive fluid (10) deposited in a pressure controlled electrochemical chamber (1) to the extent that all surfaces of the wafers are submersed.
• p-Doped Silicon is less sensitive and can usually be etched without light irradiation having a wavelength in the range of 100 to 1300 nm for preventing passivation. However, such a light irradiation will enhance the reaction.
• light irradiation is necessary for electrochemical etching unless strongly reactive solutions are employed that affect the passivation. Only low doped n-type silicon can be etched with bigger rates without extra irradiation. Usually the etching rate of n-type silicon is lower than the etching rate of p- doped silicon at identical etching conditions.
• the etching conditions are:
• a) for p-doped silicon in plane 100 a voltage of about +0.6 V over NHE (normal
• n-doped silicon wafers a voltage of about 0.02 V plus light irradiation at 100 to 1300 nm.
• the conductive fluid used is preferably 1 M HCI in aqueous solution.
• the typical etch rate achieved for p-doped silicon in plane 100 is about 2.4 ⁇ /min at 95 °C.
• the electrochemical etching (ECE) results in porous silicon (p-Si, HF & ECE or n-Si, HF, ECE S light, 1.0 M HF).
• Solar cells need a clean wafer surface.
• a typical metal concentration is 10 +12 to 10 +15 atoms/cm 2 and this figure should be reduced by a factor 1000 for critical metals like Cu.
• Cu is more noble than Si, so it will deposit first at the cathode.
• the reactions taking place at the anode during electrochemical etching are typically:
• the cleaning action can be further enhanced by reversing the polarity, thus repelling cationic contaminants from the cathode and anionic contaminants from the anode.
• the wafers in the array can be alternatingly contacted to anodic/cathodic voltage to reduce the shielding effect in the one-anode wafer- stack.
• the wafer stack can function as further cathode and anode simultaneously.
• the wafers can be attached to the glas beam by means of conductive glue which is similar to a common LCD panel contact glued to a PCB with alternating poles.
• conductive glue is similar to a common LCD panel contact glued to a PCB with alternating poles.
• An anisotropic conductive glue is preferred.
• the electrochemical processing does not require any separate anode.
• the alternating polarity can be kept, the polarity can be reversed one or several times. In this alternating charge wafer arrangement the bubbles and the high diffusion rate of the solution carries out the impurities.
• the frequency of alternating the charge is limited by the capacitor effect of the electrical double layer: surface-anodic to solution and solution to surface-cathodic.
• the advantage of the alternating wafer charge mode is that cationic (e.g. metals) as well as anionic (e.g. halogen) contaminants are removed effectively.
• a separately located cathode located outside the wafer stack is present, preferably having the most negative potential take up all the impurities in the fluid medium, thereby having a cleaning effect on contaminants.

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