WO2009142704A1 - Method of etching - Google Patents

Method of etching Download PDF

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Publication number
WO2009142704A1
WO2009142704A1 PCT/US2009/002992 US2009002992W WO2009142704A1 WO 2009142704 A1 WO2009142704 A1 WO 2009142704A1 US 2009002992 W US2009002992 W US 2009002992W WO 2009142704 A1 WO2009142704 A1 WO 2009142704A1
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WIPO (PCT)
Prior art keywords
channel
etchant
channels
liquid
region
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PCT/US2009/002992
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French (fr)
Inventor
Christopher Lee Bower
Christopher Barrie Rider
Katalin Hegedus
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Eastman Kodak Company
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Publication of WO2009142704A1 publication Critical patent/WO2009142704A1/en

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    • 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
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/31—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers
    • H01L21/3205—Deposition of non-insulating-, e.g. conductive- or resistive-, layers on insulating layers; After-treatment of these layers
    • H01L21/321—After treatment
    • H01L21/3213—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer
    • H01L21/32133—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer by chemical means only
    • H01L21/32134—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer by chemical means only by liquid etching 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
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/027—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34
    • H01L21/033—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers
    • H01L21/0334—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane
    • 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
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/31—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers
    • H01L21/3205—Deposition of non-insulating-, e.g. conductive- or resistive-, layers on insulating layers; After-treatment of these layers
    • H01L21/321—After treatment
    • H01L21/3213—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer
    • H01L21/32139—Physical or chemical etching of the layers, e.g. to produce a patterned layer from a pre-deposited extensive layer using masks
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components 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
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14683—Processes or apparatus peculiar to the manufacture or treatment of these devices or parts thereof
    • 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

Definitions

  • the invention relates to a patterning method and in particular to a method of patterning a surface in the manufacture of devices and structures for electronic, optical and optoelectronic, sensing and security applications.
  • the invention also relates to devices and structures manufactured by the method and to patterning surfaces and substrates on which devices and structures can be manufactured.
  • TFT thin-film transistor
  • TFTs can also be applied to the production of sensors, such as chemical sensor and biosensor arrays and other optoelectronic devices such as photovoltaics, photodetector arrays for scanning applications or image capture and organic light emitting diode arrays for electronic displays or image sensors.
  • sensors such as chemical sensor and biosensor arrays and other optoelectronic devices such as photovoltaics, photodetector arrays for scanning applications or image capture and organic light emitting diode arrays for electronic displays or image sensors.
  • additive processes offer great promise as patterned deposition technologies particularly where materials can be deposited in liquid form.
  • additive processes such as conventional printing and inkjet have more limited applicability.
  • InkJet droplet sizes are typically of the order of a few picolitres.
  • a 1 pi droplet has an in-flight diameter of 12 microns.
  • US 2005/0170550 describes the use of banks of appropriate wettability formed on a surface to contain liquid droplets that are incident on the surface between a pair of banks. The process for forming the banks and for profiling the wettability of the sides requires several steps.
  • US 71 15507 describes a method of restricting the lateral spreading of a liquid droplet on a surface by the use of indent regions.
  • vapour phase it is sometimes preferable to deposit materials from the vapour phase and additive liquid processes are not then possible.
  • zinc oxide is a promising material for the semiconductor layer of a TFT, but the electron mobility is found to be higher in materials deposited, not by solution based processes, but by vapour based processes, such as sputtering or atomic layer deposition (ALD).
  • vapour based processes such as sputtering or atomic layer deposition (ALD).
  • Many vapour based processes including sputtering, ALD, chemical vapour deposition (CVD) and thermal evaporation, do not lend themselves to direct additive deposition of materials only where they are needed.
  • etch it is therefore necessary to remove material from a coated layer where it is not needed, by some form of etch: a dry etch like for example a reactive ion etch (RIE) or a wet etch using corrosive liquids. Removal of material is also desirable when topological features are being constructed, for example, to create micromechanical structures, diffractive and refractive micro-optics. Patterning using an etch step is usually achieved using additional processes to provide a patterned etch mask such as photolithography to pre-pattern the substrate before deposition or to post-pattern the coated layer after deposition. Etch masks may also be deposited using printing processes.
  • RIE reactive ion etch
  • a further need for removal of materials occurs with the requirement to create holes through layers so that connections can be made from an upper layer to a lower layer through a non-conductive layer.
  • These additional patterning steps are time consuming and require registration with other previously patterned device features. It is also wasteful of materials to have to cover the whole surface with an etch resist such that material is only removed where desired during the etching step.
  • Liquid etch solutions are generally highly corrosive and toxic, so it is desirable from an environmental standpoint to reduce the volume of etch solution required.
  • etch solutions may damage materials other than the layer to be removed and it is often required that the liquid etch must only come into contact with the layer which is to be patterned. Splashing of liquid etches must also be avoided during the local application of the etch to the area to be etched, otherwise defects may be created.
  • An efficient, well controlled method of patterned etching without using photolithographic processes is desirable.
  • capillary forces to move liquid around in narrow closed channels is well known.
  • the use of capillary channels to pattern liquids is also known, as is evident from the rapidly expanding field of micro fluidics, see for instance Rev. Mod. Phys. 77, 977 (2005).
  • capillary channels being used to improve the resolution of established printing and patterning methods, in which the printed liquid is further confined by physical barriers to avoid undesirable spread on the substrate.
  • selective etching can be achieved by masking a region of a substrate to be patterned and dipping the entire sample into a suitable liquid etchant and using capillary rise to improve the etch quality as described in US5468338.
  • Capillary action has also been used as a means of delivering a liquid etchant to remove a sacrificial layer as part of a lithographic patterning process.
  • a liquid etchant is brought into contact with a sacrificial-layer and a surface-layer by narrow trenches. The surface-layer is then undercut and the sacrificial-layer is selectively removed by the etchant.
  • the etching process in this method is unconstrained by the trench, which merely serves to deliver the etchant into contact with the sacrificial-layer.
  • US2003/01 16535 describes a method and apparatus for removing coating layers from aligning marks by placing the sample onto a spin processor and creating a narrow gap that draws etchant in by capillary action. Again, the etchant is unconstrained within the gap, but some degree of control of the wetting line position is achieved by control of the rotational speed of the sample on the spin processor, by balancing the capillary force with centripetal force.
  • the present invention solves the problems of how to fabricate microstructures at high resolution by the selective etching of thin layers using a minimal volume of etch liquid or "etchant", in a manner which is self-aligning wherein the etch liquid comes into contact only with the regions of the layer which are to be removed.
  • a method of selectively removing material from a surface comprising the steps of: a) providing the surface with at least one open channel, wherein the channel defines the region where material is to be removed b) depositing a flowable etchant adjacent to the said at least one open channel, such that the etchant is drawn into the channel by capillary forces wherein material is transferred from the surface into the etchant.
  • a flowable etchant adjacent to the said at least one open channel, such that the etchant is drawn into the channel by capillary forces wherein material is transferred from the surface into the etchant.
  • the substrate may have preformed channels, or they may be created by embossing the substrate, or by using a photo-resist or other patterned material to define the channels.
  • the channels may be formed by printing suitable material by inkjet, flexographic printing, screen- printing, gravure printing or some other patterned deposition technique.
  • the thin films may be deposited by any convenient method, e.g. by a vapour coating technique, by a liquid coating technique or by printing or spray methods.
  • the thin films may be deposited either before or after the substrate is provided with the channel.
  • the surface of the thin film which is presented within the channel may be etched to create the desired pattern.
  • the liquid etchant is confined to the channel, so that only material within the channel may be removed.
  • the movement of the liquid along the channel also provides renewal of the etch solution at the surface of the material being etched, so only a small volume of liquid etchant is required.
  • the method can be used to pattern a thin film over a large area at high resolution and at low cost by wet etching using a minimal amount of etchant and only allowing the etchant to contact the thin film in the regions where etching is required.
  • the uppermost layer may itself be resistant to the etchant but may also be porous such that the etchant penetrates through the uppermost layer and etches an underlying layer such that the etch resistant layer may be removed.
  • etching a surface includes etching either the surface of the uppermost layer within the channel or etching material below the upper surface, including in other layers below the uppermost layer. In this latter case, the etchant is able to access the material below the upper surface due to the porosity of the upper layers.
  • the present invention avoids the requirement for the entire sample to be immersed in a bath of the liquid etch by only delivering the liquid etch to the regions where material is to be removed. By using capillary forces to deliver an etch liquid only to where material is to be removed, the volume of etchant required is greatly reduced.
  • renewal of the etch liquid at the surface of the material being etched is normally provided by mixing or agitating the sample.
  • continuous renewal of the etch liquid in contact with the surface of the material to be etched is provided by the rolling motion of the liquid as the wetting-line advances along the channel.
  • the etch liquid may be deposited by a standard delivery method such as a micro-droplet generator, or inkjet printer that delivers the required quantity of liquid to a single, or small number of, reservoir regions on the substrate. Since the reservoir regions are minimized, the propensity for the etch liquid to be deposited in unwanted regions of the substrate is also reduced. If a photolithographic, embossing or micro-replication process is used to pattern the channels, the width of the channels may be below that normally obtained by standard printing methods, thus allowing very high resolution features to be etched. Since the volume of etch liquid required is small this reduces the amount of waste and the environmental impact of the technique compared to others which involve immersing the entire sample within a liquid etch bath.
  • a standard delivery method such as a micro-droplet generator, or inkjet printer that delivers the required quantity of liquid to a single, or small number of, reservoir regions on the substrate. Since the reservoir regions are minimized, the propensity for the etch liquid to be deposited in unwanted regions of the substrate is also
  • Figure Ia shows a plan view of a substrate having a series of circular channels with square cross-section in which a metal layer to be selectively etched is underneath the channels;
  • Figure Ib shows a side view of the substrate of Figure 1 a;
  • Figure 2a shows a plan view of a substrate having a series of circular channels with square cross-section in which a metal layer to be selectively removed is a conformal coating on top of the channels;
  • Figure 2b shows a side view of the substrate of Figure 2a
  • Figure 3a shows a plan view of a substrate having linear channels with a
  • Figure 3b shows a side view of the substrate of Figure 3a.
  • the present invention relates to a patterning method and in particular to a method of patterning a surface in the manufacture of devices and structures for electronic, optical and optoelectronic, sensing and security applications.
  • the invention also relates to devices and structures manufactured by the method and to patterning surfaces and substrates on which devices and structures can be manufactured.
  • a flowable etchant is drawn along channels by capillary forces to remove material from all or part of the channel surface.
  • the surface which is to be patterned can be the same as the substrate material or it can be a thin film or multiple layers of thin films comprised of materials different from that of the substrate.
  • the substrate can have a series of channels embossed into the surface.
  • the surface of the channels can then be coated with a conformal film or films of metal or inorganic material by evaporation or sputter coating, or the surface may be coated with a conformal polymer layer deposited from solution or by evaporation.
  • the coating is generally thin compared to the depth of the channels so that the morphology of the channels is essentially unchanged by the application of the thin film layer or layers.
  • the thickness of the surface which is removed is generally small compared to the depth of the channels, since the volume of liquid etchant involved is also small, so that the morphology of the channels is essentially unchanged after removal of the surface material. It is preferable that the thickness of material removed is less than 1 micrometer but in cases where a continuous flow of etchant along the channel is provided, the thickness of material removed may be greater.
  • the flowable etchant dissolves the material at the surface, such that material is removed from the solid state into the etchant liquid.
  • the liquid etchant will completely dissolve the surface material.
  • the preferred liquid etchant is a solvent for the polymer. It is not necessary for the liquid to completely dissolve the material at the surface.
  • the material is a composite, it is possible to dissolve the binder and release solid particles into the etchant liquid.
  • the surface is a thin film of material, such as a polymer layer, it is sufficient for the etchant liquid to undercut the layer thereby promoting de-lamination of the layer by subsequent physical process such as abrasion washing or peeling.
  • the preferred etchant is an acid, which dissolves the metal by solvation of the constituent ions in the acid solution. It is not necessary for the liquid etchant to be a solvent for the material to be removed. Some materials, such as low melting point waxes for example, can be removed by using a hot liquid as the etchant. In this case the surface material is melted when it comes into contact with the hot etchant and may be removed from the channel together with the etchant.
  • etchant liquid comes into contact with the channel or channels, capillary forces cause it to fill the channel. In this manner the material on the surface within the channel region may be removed from the surface by the etchant liquid. If the etchant liquid is not itself taken out the channel by various methods as described below, the material which has been removed from the surface and transferred into the etchant can be re-deposited within the channel region, unless the material can be converted into vapour and evaporated, which may be achieved for some materials by correct choice of the etchant liquid. For example a saccharide, or polythiophene layer can be removed by using an acid etch to convert it to gaseous products.
  • the re-deposited material may have bulk properties different to the original material, so that a change in the conductivity, optical properties, chemical constituents, stoichiometry or morphology may occur after re-deposition.
  • an aqueous acid etchant deposited into the channel or channels will dissolve the metal layer, converting it into solvated ions within the channel region. If there is no flux of the liquid etchant out of the channel region and the etchant is allowed to evaporate, the metal ions will be re-deposited in the channel region.
  • the morphology of the re-deposited film is no longer uniform and the resulting distribution of the metal can cause a significant reduction in the conductivity of the metal layer.
  • the optical properties of the metal layer can also be changed from predominately reflective to predominately scattering.
  • a 'reservoir' region which connects with the channel somewhere along its length, and a 'sump' region which connects with the channel at some other point along its length.
  • Multiple reservoirs and sumps may be provided depending on the channel length and volume.
  • the reservoir region is generally a depression created in the surface adjacent to the channel or channels, to contain the deposited etchant liquid, such that flowable liquid etchant deposited there will be drawn into the channel by capillary forces.
  • the reservoir region may also be a through-hole in the substrate which connects to the channel at the surface to be patterned so that etchant which is deposited on the other side of the substrate is drawn up through the hole by capillary forces and then flows into the channel upon reaching it.
  • the sump region is a similar depression located at the end of the channel or channels to receive the liquid etchant after flow through the channels, so that the etchant liquid can be effectively removed from the channel.
  • To fully empty a channel the receding contact angle of the liquid within the channel must be non-zero to avoid pinning of the liquid. Removal of the etchant liquid from the channel can be achieved by use of suction.
  • This approach may be achieved for certain channel geometries by bringing a suction tube into contact with the liquid surface in the channel or preferably with the liquid surface in the sump region. More preferably this approach may be facilitated by the use of a suction head brought into very close proximity to the channel and preferably to the sump region, such that liquid is pulled off the sump region and into the suction head by the strong air flow into the head. As the sump region is emptied, etchant is drawn out of the channel region and into the sump by capillary forces.
  • a combined approach of an air knife which blows liquid out of the sump and channel regions and a suction head which uses a strong airflow to draw all the liquid droplets which have been blown off the surface by the air knife directly into it may also be used. Removal of the etchant from the channel, sump or reservoir regions may also be achieved when a porous or absorbent material is brought into contact with the entirety of the channel or a subsection of the channel or the sump or reservoir regions to remove the etchant liquid by capillary action.
  • the sump region can also be a through-hole at the end of the channel such that the etchant liquid is drawn from the channel and into the through-hole to allow it to be removed on the opposite side of the substrate, by either a vacuum, porous or absorbent material, or using another channel or channels adjacent to the through-hole.
  • This approach is particularly advantageous in that smearing or inadvertent splashing of etchant as it is removed does not damage the patterned surface.
  • the etchant liquid can be deposited by any suitable dispensing method, but additive printing methods are preferred. Additive printing methods include, inkjet printing using either a drop-on-demand or a continuous droplet generation device, flexographic, gravure, offset or intaglio printing, screen printing and pad printing. A further processing step of flushing the channel with a low surface tension solvent may also be performed to prevent further etching of the material within the channel region, or aid removal of etched material from within the channel after the etch is complete.
  • Example 1 Etching of metallic films beneath capillary channels.
  • Figure Ia shows a plan view of a substrate having a series of channels in which a metal layer to be selectively etched is underneath the channels.
  • Figure Ib shows a cross section of the substrate along the line AB in Figure Ia.
  • a glass substrate 5 was provided with a layer 4 of either silver or aluminium, lOOnm thick, by vacuum evaporation.
  • Capillary channels 2 were created on top of the glass substrate 5.
  • the channels were created by patterning a layer of 50 micrometer thick LaminarTM photoresist 3 using standard lithographic techniques.
  • the channels 2 were created with a range of widths but generally a width of 100-300 micrometers was used.
  • a pattern as shown in Figure Ia was created. Each channel was in contact with a reservoir region 1 and a sump region 6.
  • the channels had a square cross-section and were open at the bottom, so that any liquid in the channel was in direct contact with the metal layer at the bottom of the channel.
  • a liquid etchant was introduced into the channel at the reservoir region 1 and was drawn along the channel by capillary forces, so that eventually the entire channel was filled with liquid etchant.
  • the contact angle of the liquid etchant on the interior surface of the channel should be as low as possible and generally at least less than 90 degrees.
  • the majority of liquid etchants that can be used for removal of metal are aqueous solutions of concentrated acids which due to the high contact angle usually prevents effective filling of the channels by capillary forces.
  • a surfactant was used to lower the contact angle ofthe etchant on the channels.
  • the channels were first wetted with the surfactant by using a non-aqueous solvent such as ethanol or acetone to carry the surfactant into the channels.
  • a non-aqueous solvent such as ethanol or acetone
  • OHn 1OG was used as the surfactant to increase the wettability of the channels but other surfactants could be used for this purpose.
  • a droplet of the surfactant solution was deposited in a reservoir region by pipette and was drawn along the channels by capillary action leaving a thin layer of surfactant on the surface of the channels after evaporation of the solvent.
  • a concentrated acid etch such as hydrochloric acid (HCl), nitric acid (HNO 3 ), sulphuric acid (H 2 SO 4 ), chromic acid (K 2 Cr 2 O 7 ), and phosphoric acid (H 3 PO 4 )
  • HCl hydrochloric acid
  • HNO 3 nitric acid
  • SO 4 sulphuric acid
  • chromic acid K 2 Cr 2 O 7
  • phosphoric acid H 3 PO 4
  • Figure 2a shows another embodiment in which the substrate 25 already has a series of capillary channels 22 patterned on the surface with a conformal metal coating 24 on top of the channels.
  • the channels may be formed using photo-resist 23, as in the previous example, or may be formed by embossing, moulding or micro-replication. In this case the channel defines where the metal is to be removed since the etchant is confined within the channel.
  • Figure 2b shows a cross-section through the substrate along the line AB
  • the channels were either formed from photo-resist patterned on a glass substrate, having a square cross section as in Figure 2a, or were created by microreplication in a plastic or polymeric substrate resulting in channels with a 'V shaped cross section as shown in Figure 3a.
  • a sample of polycarbonate with 'V shaped channels coated with a layer of either silver or aluminium approximately lOOnm thick was selectively etched using various concentrated acid solutions: hydrochloric acid (HCl), nitric acid (FTNO 3 ), sulphuric acid (H 2 SO 4 ), chromic acid (K 2 Cr 2 O 7 ), and phosphoric acid (H 3 PO 4 ).
  • HCl hydrochloric acid
  • FTNO 3 nitric acid
  • sulphuric acid H 2 SO 4
  • chromic acid K 2 Cr 2 O 7
  • phosphoric acid H 3 PO 4
  • the surface energy of the channels was increased by first wicking surfactant 1OG along the channels using a low surface tension liquid such as ethanol or acetone.
  • approximately 1 microlitre of the etchant liquid was placed into the reservoir region 26 of the channel to be etched, and the liquid was drawn along the channel by capillary action.
  • the rinse steps can be neglected, since although metal is re-deposited within the channels, the re-deposited layer is not continuous or uniform so the electrical conductivity is greatly reduced.
  • Example 3 Etching of inorganic layers beneath channels
  • capillary channels were created on top of a glass substrate 5 with an inorganic layer 4 of either zinc oxide or indium tin oxide (ITO) of order lOOnm thick , by patterning a layer of photoresist 50 micrometer thick 3 using standard lithographic techniques.
  • Channels 2 were created with a range of widths but generally a width of 100-300 micrometers was used.
  • a pattern as shown in Figure 1 was created. The channels had a square cross-section and were open at the bottom, so that any liquid in the channel was in direct contact with the inorganic layer at the bottom of the channel.
  • a liquid etchant was introduced into the reservoir region 1 and was drawn along the channel by capillary forces, so that eventually the entire channel was filled with liquid etchant.
  • the contact angle of the liquid etchant on the interior surface of the channel should be as low as possible and generally at least less than 90 degrees.
  • the majority of liquid etchants that can be used for removal of inorganic oxide materials are aqueous solutions of concentrated acids, which due to the high contact angle, usually prevents effective filling of the channels by capillary forces.
  • a surfactant flush was used to lower the contact angle of the etchant on the channels as described in example 1.
  • a surfactant was used to pre- wett the channels by using a non-aqueous solvent such as ethanol or acetone to carry the surfactant into the channels.
  • a non-aqueous solvent such as ethanol or acetone
  • OHn 1 OG was used as the surfactant to increase the wettability of the channels but other surfactants could be used for this purpose.
  • a concentrated acid etch from the following list were deposited into the reservoir regions 1 of the various patterned areas and was driven along the channels by capillary action: oxalic acid (C 2 H 2 O 4 ), acetic acid (CH 3 COOH), hydrochloric acid (HCl), nitric acid (HNO 3 ), sulphuric acid (H 2 SO 4 ), chromic acid (K 2 Cr 2 O 7 ) and phosphoric acid (H 3 PO 4 ).
  • the circulation of fresh etchant at the surface of the inorganic material was ensured due to the rolling motion present at the advancing contact line of the liquid etchant.
  • the channels were either formed from photo-resist patterned on a glass or flexible polymer substrate, having a square cross section or were created by microreplication in a plastic or polymeric substrate resulting in channels with a 'V shaped cross section.
  • Example 4 Etching of conformal inorganic layers beneath channels.
  • a sample of polycarbonate 35 with 'V shaped channels 32 with a coating of either zinc oxide or indium tin oxide 34 which was approximately lOOnm thick was selectively etched using a concentrated acid solution from the list: oxalic acid (C 2 H 2 O 4 ), acetic acid (CH 3 COOH), hydrochloric acid (HCl), nitric acid (HNO 3 ), sulphuric acid (H 2 SO 4 ), chromic acid (K 2 Cr 2 O 7 ), and phosphoric acid (H 3 PO 4 ).
  • the surface energy of the channels was increased by first wicking surfactant 1OG along the channels using a low surface tension liquid such as ethanol or acetone.
  • a low surface tension liquid such as ethanol or acetone.
  • approximately 1 microlitre of the etchant liquid was placed into the channel to be etched and the liquid was driven along the channel by capillary action.
  • a delay of several seconds or minutes was allowed, depending upon the etchant being used, before the etchant was removed from the channels by application of suction or a capillary wicking material to the channel.
  • a further processing step of flushing the channel with solvent could be performed to prevent further etching of the metal at the base of the channel.
  • a solvent was applied to one end of the channel and subsequently removed from the other end of the channel.
  • Example 5 Etching of polymer layers
  • capillary channels were created on top of a glass substrate 5 with a layer 4 of conductive polythiophene polymer (Baytron F CPP105M, Bayer) of order lOOnm thick, by patterning a layer of 50 micrometer thick photoresist 3 using standard lithographic techniques. A pattern as shown in Figure 1 was created. The channels had a square cross-section and were open at the bottom, so that any liquid in the channel was in direct contact with the polymer layer at the bottom of the channel.
  • conductive polythiophene polymer Bayer
  • a liquid etchant was introduced into the reservoir region 1 and was drawn along the channel by capillary forces, so that eventually the entire channel was filled with liquid etchant.
  • a delay of several seconds or minutes was allowed, depending upon the etchant being used, before the etchant was removed from the channels by application of suction or a capillary wicking material in the sump region 6. If required, a further processing step of flushing the channel with another solvent could be performed to prevent further etching of the polymer at the base of the channel.
  • the channels were either formed from photo-resist patterned on a glass or flexible polymer substrate, having a square cross section or were created by microreplication in a plastic or polymeric substrate resulting in channels with a 'V shaped cross section.
  • Hexan-1-ol, Octan-1-ol and other longer chain alcohols were found to be most effective etchants for the polythiophene layer, since they had sufficiently low surface tension to promote wicking and were a suitable solvent for the polymer layer.
  • the polymer layer Before etching, had a resistance of order 0.2 Mega Ohms when measured between the inside and outside of the circular channels 2 in Figure 1. After etching the resistance rose by an order of magnitude to more than 2 Mega Ohms.

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Abstract

A method of selectively removing material from a surface, the method comprising the steps of: providing the surface with at least one open channel, the channel defining the region where material is to be removed; and depositing a flowable etchant adjacent to the said at least one open channel, such that the etchant is drawn into the channel by capillary forces, material being transferred from the surface into the etchant.

Description

METHOD OF ETCHING
FIELD OF THE INVENTION
The invention relates to a patterning method and in particular to a method of patterning a surface in the manufacture of devices and structures for electronic, optical and optoelectronic, sensing and security applications. The invention also relates to devices and structures manufactured by the method and to patterning surfaces and substrates on which devices and structures can be manufactured.
BACKGROUND OF THE INVENTION
The development of silicon-based thin-film transistor (TFT) technology has been an essential enabler for the development of large flat panel displays. Despite the huge cost of factories to manufacture TFTs on glass and the complexity of the TFT manufacturing process, the technology is now well- established for active matrix liquid crystal displays and is based largely on photolithographic techniques for depositing patterns of the various materials into multilayer structures.
In recent years great progress has been made on TFT technologies based on other semiconductors including polymers, metal oxides and semiconducting nano wires and nano tubes. Many of these approaches benefit from simpler processes that promise greatly reduced capital investment compared to the current silicon-based factories which use photolithography for patterning of features. Another recent development has seen liquid processed semiconductors deposited onto flexible substrates using additive approaches such as inkjet and conventional printing and this promises further process simplification and cost-reduction as more processes can be performed in roll-to-roll configurations.
These new approaches to the production of TFTs can also be applied to the production of sensors, such as chemical sensor and biosensor arrays and other optoelectronic devices such as photovoltaics, photodetector arrays for scanning applications or image capture and organic light emitting diode arrays for electronic displays or image sensors. For large feature sizes, additive processes offer great promise as patterned deposition technologies particularly where materials can be deposited in liquid form. For small feature sizes, however, additive processes, such as conventional printing and inkjet have more limited applicability. InkJet droplet sizes are typically of the order of a few picolitres. A 1 pi droplet has an in-flight diameter of 12 microns. When it lands it spreads and depending on the surface energy of the substrate and the surface tension of the liquid, the diameter of the circle that is now covered with liquid could be much larger than the diameter of the original droplet. This significantly limits the resolution of patterning that can be achieved. Furthermore, there is a limit to the accuracy with which inkjet droplets can be placed at a precise location on a surface, and the thickness of liquid that can be deposited.
Many approaches have been suggested to reduce the pattern resolution limitations arising from inkjet droplet sizes. US 2005/0170550 describes the use of banks of appropriate wettability formed on a surface to contain liquid droplets that are incident on the surface between a pair of banks. The process for forming the banks and for profiling the wettability of the sides requires several steps. US 71 15507 describes a method of restricting the lateral spreading of a liquid droplet on a surface by the use of indent regions. These approaches suffer from added complexity.
It is sometimes preferable to deposit materials from the vapour phase and additive liquid processes are not then possible. For example, zinc oxide is a promising material for the semiconductor layer of a TFT, but the electron mobility is found to be higher in materials deposited, not by solution based processes, but by vapour based processes, such as sputtering or atomic layer deposition (ALD). Many vapour based processes, including sputtering, ALD, chemical vapour deposition (CVD) and thermal evaporation, do not lend themselves to direct additive deposition of materials only where they are needed. It is therefore necessary to remove material from a coated layer where it is not needed, by some form of etch: a dry etch like for example a reactive ion etch (RIE) or a wet etch using corrosive liquids. Removal of material is also desirable when topological features are being constructed, for example, to create micromechanical structures, diffractive and refractive micro-optics. Patterning using an etch step is usually achieved using additional processes to provide a patterned etch mask such as photolithography to pre-pattern the substrate before deposition or to post-pattern the coated layer after deposition. Etch masks may also be deposited using printing processes. A further need for removal of materials occurs with the requirement to create holes through layers so that connections can be made from an upper layer to a lower layer through a non-conductive layer. These additional patterning steps are time consuming and require registration with other previously patterned device features. It is also wasteful of materials to have to cover the whole surface with an etch resist such that material is only removed where desired during the etching step. Liquid etch solutions are generally highly corrosive and toxic, so it is desirable from an environmental standpoint to reduce the volume of etch solution required. Furthermore, etch solutions may damage materials other than the layer to be removed and it is often required that the liquid etch must only come into contact with the layer which is to be patterned. Splashing of liquid etches must also be avoided during the local application of the etch to the area to be etched, otherwise defects may be created. An efficient, well controlled method of patterned etching without using photolithographic processes is desirable.
When fabricating a multi-layer TFT device, accurate registration between the features in all the layers is very important if optimum device performance is to be achieved. Certain features, however, require greater alignment accuracy than others. Alignment of the gate electrode with the channel region of a TFT formed between the source and drain electrodes is very important and overlap between the gap and source and drain electrodes is to be avoided. On the other hand, the semiconductor and dielectric layers may significantly overlap the device electrodes without detriment to performance, provided that adequate isolation is achieved between neighbouring TFTs and between neighbouring pixels so that leakage currents do not cause inter-pixel cross-talk. Thus, some features of a TFT require accurate and high-resolution pattern registration, while others do not. During a TFT manufacturing process many separate steps are typically required. Between each step and even during a step, environmental conditions such as ambient temperature and humidity may change, causing changes in dimensions of the carrier substrate for the TFTs and of masks or positioning equipment for deposition or patterning tools. Thus registration errors may accumulate between TFT layers and this can lead to poor device performance. The temperature of the substrate and deposition or patterning tools may also change during an individual process step as sources of heat are frequently necessary for deposition, patterning or post deposition treatments. These temperature changes also cause dimensional changes within a single patterned layer and again registration may be affected. There is therefore a need to accurately control registration and alignment as well as the dimensions of individual patterns throughout the manufacturing process of thin film optoelectronic devices in general.
The use of capillary forces to move liquid around in narrow closed channels is well known. The use of capillary channels to pattern liquids is also known, as is evident from the rapidly expanding field of micro fluidics, see for instance Rev. Mod. Phys. 77, 977 (2005). There are also examples of capillary channels being used to improve the resolution of established printing and patterning methods, in which the printed liquid is further confined by physical barriers to avoid undesirable spread on the substrate. It is also known that selective etching can be achieved by masking a region of a substrate to be patterned and dipping the entire sample into a suitable liquid etchant and using capillary rise to improve the etch quality as described in US5468338. In many situations however it is desirable to be able to selectively etch regions of a substrate such that the etchant only comes into contact with the regions to be etched. In this case, etchant must be delivered only to those regions and not be allowed to contact the entire sample. US6153532 describes a means of depositing etchant only in the prescribed regions of a sample by using a movable etchant applicator tip which is brought close enough to the sample to transfer a small quantity of liquid etchant at the desired location.
Capillary action has also been used as a means of delivering a liquid etchant to remove a sacrificial layer as part of a lithographic patterning process. In US5710057 a liquid etchant is brought into contact with a sacrificial-layer and a surface-layer by narrow trenches. The surface-layer is then undercut and the sacrificial-layer is selectively removed by the etchant. The etching process in this method is unconstrained by the trench, which merely serves to deliver the etchant into contact with the sacrificial-layer. US2003/01 16535 describes a method and apparatus for removing coating layers from aligning marks by placing the sample onto a spin processor and creating a narrow gap that draws etchant in by capillary action. Again, the etchant is unconstrained within the gap, but some degree of control of the wetting line position is achieved by control of the rotational speed of the sample on the spin processor, by balancing the capillary force with centripetal force.
PROBLEM TO BE SOLVED
The present invention solves the problems of how to fabricate microstructures at high resolution by the selective etching of thin layers using a minimal volume of etch liquid or "etchant", in a manner which is self-aligning wherein the etch liquid comes into contact only with the regions of the layer which are to be removed.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of selectively removing material from a surface, the method comprising the steps of: a) providing the surface with at least one open channel, wherein the channel defines the region where material is to be removed b) depositing a flowable etchant adjacent to the said at least one open channel, such that the etchant is drawn into the channel by capillary forces wherein material is transferred from the surface into the etchant. In the manufacture of micro-patterned devices or microstructures it is frequently necessary to pattern thin films which are deposited onto a substrate. A channel or channels is created upon the substrate, the channels being of fine enough resolution such that a liquid etch can be drawn along the channel by capillary forces. The substrate may have preformed channels, or they may be created by embossing the substrate, or by using a photo-resist or other patterned material to define the channels. In another embodiment the channels may be formed by printing suitable material by inkjet, flexographic printing, screen- printing, gravure printing or some other patterned deposition technique. The thin films may be deposited by any convenient method, e.g. by a vapour coating technique, by a liquid coating technique or by printing or spray methods. The thin films may be deposited either before or after the substrate is provided with the channel. The surface of the thin film which is presented within the channel may be etched to create the desired pattern. The liquid etchant is confined to the channel, so that only material within the channel may be removed. The movement of the liquid along the channel also provides renewal of the etch solution at the surface of the material being etched, so only a small volume of liquid etchant is required. The method can be used to pattern a thin film over a large area at high resolution and at low cost by wet etching using a minimal amount of etchant and only allowing the etchant to contact the thin film in the regions where etching is required.
In the manufacture of microstructures it is common to use multiple layers of different materials. There may be for example several thin film layers of material within the channel region formed on the supporting substrate. The uppermost layer may itself be resistant to the etchant but may also be porous such that the etchant penetrates through the uppermost layer and etches an underlying layer such that the etch resistant layer may be removed.
It will be understood by someone who is skilled in the art that the term "etching a surface" includes etching either the surface of the uppermost layer within the channel or etching material below the upper surface, including in other layers below the uppermost layer. In this latter case, the etchant is able to access the material below the upper surface due to the porosity of the upper layers.
ADVANTAGEOUS EFFECT OF THE INVENTION
The present invention avoids the requirement for the entire sample to be immersed in a bath of the liquid etch by only delivering the liquid etch to the regions where material is to be removed. By using capillary forces to deliver an etch liquid only to where material is to be removed, the volume of etchant required is greatly reduced. In a traditional liquid etching process, renewal of the etch liquid at the surface of the material being etched is normally provided by mixing or agitating the sample. In the present invention, continuous renewal of the etch liquid in contact with the surface of the material to be etched is provided by the rolling motion of the liquid as the wetting-line advances along the channel. Furthermore, the etch liquid may be deposited by a standard delivery method such as a micro-droplet generator, or inkjet printer that delivers the required quantity of liquid to a single, or small number of, reservoir regions on the substrate. Since the reservoir regions are minimized, the propensity for the etch liquid to be deposited in unwanted regions of the substrate is also reduced. If a photolithographic, embossing or micro-replication process is used to pattern the channels, the width of the channels may be below that normally obtained by standard printing methods, thus allowing very high resolution features to be etched. Since the volume of etch liquid required is small this reduces the amount of waste and the environmental impact of the technique compared to others which involve immersing the entire sample within a liquid etch bath.
DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example with reference to the following drawings in which:
Figure Ia shows a plan view of a substrate having a series of circular channels with square cross-section in which a metal layer to be selectively etched is underneath the channels; Figure Ib shows a side view of the substrate of Figure 1 a;
Figure 2a shows a plan view of a substrate having a series of circular channels with square cross-section in which a metal layer to be selectively removed is a conformal coating on top of the channels;
Figure 2b shows a side view of the substrate of Figure 2a; Figure 3a shows a plan view of a substrate having linear channels with a
'V cross-section in which the a metal layer to be selectively removed is a conformal coating on top of the channels; and
Figure 3b shows a side view of the substrate of Figure 3a.
DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a patterning method and in particular to a method of patterning a surface in the manufacture of devices and structures for electronic, optical and optoelectronic, sensing and security applications. The invention also relates to devices and structures manufactured by the method and to patterning surfaces and substrates on which devices and structures can be manufactured. A flowable etchant is drawn along channels by capillary forces to remove material from all or part of the channel surface.
The surface which is to be patterned can be the same as the substrate material or it can be a thin film or multiple layers of thin films comprised of materials different from that of the substrate. For instance the substrate can have a series of channels embossed into the surface. The surface of the channels can then be coated with a conformal film or films of metal or inorganic material by evaporation or sputter coating, or the surface may be coated with a conformal polymer layer deposited from solution or by evaporation. The coating is generally thin compared to the depth of the channels so that the morphology of the channels is essentially unchanged by the application of the thin film layer or layers. The thickness of the surface which is removed is generally small compared to the depth of the channels, since the volume of liquid etchant involved is also small, so that the morphology of the channels is essentially unchanged after removal of the surface material. It is preferable that the thickness of material removed is less than 1 micrometer but in cases where a continuous flow of etchant along the channel is provided, the thickness of material removed may be greater.
The flowable etchant dissolves the material at the surface, such that material is removed from the solid state into the etchant liquid. In some cases the liquid etchant will completely dissolve the surface material. For instance, if the surface is a polymer or a polymer layer, the preferred liquid etchant is a solvent for the polymer. It is not necessary for the liquid to completely dissolve the material at the surface. For instance, if the material is a composite, it is possible to dissolve the binder and release solid particles into the etchant liquid. If the surface is a thin film of material, such as a polymer layer, it is sufficient for the etchant liquid to undercut the layer thereby promoting de-lamination of the layer by subsequent physical process such as abrasion washing or peeling. When the surface is metallic, the preferred etchant is an acid, which dissolves the metal by solvation of the constituent ions in the acid solution. It is not necessary for the liquid etchant to be a solvent for the material to be removed. Some materials, such as low melting point waxes for example, can be removed by using a hot liquid as the etchant. In this case the surface material is melted when it comes into contact with the hot etchant and may be removed from the channel together with the etchant.
Once etchant liquid comes into contact with the channel or channels, capillary forces cause it to fill the channel. In this manner the material on the surface within the channel region may be removed from the surface by the etchant liquid. If the etchant liquid is not itself taken out the channel by various methods as described below, the material which has been removed from the surface and transferred into the etchant can be re-deposited within the channel region, unless the material can be converted into vapour and evaporated, which may be achieved for some materials by correct choice of the etchant liquid. For example a saccharide, or polythiophene layer can be removed by using an acid etch to convert it to gaseous products. The re-deposited material may have bulk properties different to the original material, so that a change in the conductivity, optical properties, chemical constituents, stoichiometry or morphology may occur after re-deposition. For instance, when the material to be removed is a thin metallic film, an aqueous acid etchant deposited into the channel or channels will dissolve the metal layer, converting it into solvated ions within the channel region. If there is no flux of the liquid etchant out of the channel region and the etchant is allowed to evaporate, the metal ions will be re-deposited in the channel region. The morphology of the re-deposited film is no longer uniform and the resulting distribution of the metal can cause a significant reduction in the conductivity of the metal layer. The optical properties of the metal layer can also be changed from predominately reflective to predominately scattering. To facilitate the delivery to and removal of the etchant liquid from a channel there may be provided a 'reservoir' region which connects with the channel somewhere along its length, and a 'sump' region which connects with the channel at some other point along its length. Multiple reservoirs and sumps may be provided depending on the channel length and volume. The reservoir region is generally a depression created in the surface adjacent to the channel or channels, to contain the deposited etchant liquid, such that flowable liquid etchant deposited there will be drawn into the channel by capillary forces. The reservoir region may also be a through-hole in the substrate which connects to the channel at the surface to be patterned so that etchant which is deposited on the other side of the substrate is drawn up through the hole by capillary forces and then flows into the channel upon reaching it. The sump region is a similar depression located at the end of the channel or channels to receive the liquid etchant after flow through the channels, so that the etchant liquid can be effectively removed from the channel. To fully empty a channel the receding contact angle of the liquid within the channel must be non-zero to avoid pinning of the liquid. Removal of the etchant liquid from the channel can be achieved by use of suction. This approach may be achieved for certain channel geometries by bringing a suction tube into contact with the liquid surface in the channel or preferably with the liquid surface in the sump region. More preferably this approach may be facilitated by the use of a suction head brought into very close proximity to the channel and preferably to the sump region, such that liquid is pulled off the sump region and into the suction head by the strong air flow into the head. As the sump region is emptied, etchant is drawn out of the channel region and into the sump by capillary forces. A combined approach of an air knife which blows liquid out of the sump and channel regions and a suction head which uses a strong airflow to draw all the liquid droplets which have been blown off the surface by the air knife directly into it may also be used. Removal of the etchant from the channel, sump or reservoir regions may also be achieved when a porous or absorbent material is brought into contact with the entirety of the channel or a subsection of the channel or the sump or reservoir regions to remove the etchant liquid by capillary action. The sump region can also be a through-hole at the end of the channel such that the etchant liquid is drawn from the channel and into the through-hole to allow it to be removed on the opposite side of the substrate, by either a vacuum, porous or absorbent material, or using another channel or channels adjacent to the through-hole. This approach is particularly advantageous in that smearing or inadvertent splashing of etchant as it is removed does not damage the patterned surface.
The etchant liquid can be deposited by any suitable dispensing method, but additive printing methods are preferred. Additive printing methods include, inkjet printing using either a drop-on-demand or a continuous droplet generation device, flexographic, gravure, offset or intaglio printing, screen printing and pad printing. A further processing step of flushing the channel with a low surface tension solvent may also be performed to prevent further etching of the material within the channel region, or aid removal of etched material from within the channel after the etch is complete.
Example 1. - Etching of metallic films beneath capillary channels.
Figure Ia shows a plan view of a substrate having a series of channels in which a metal layer to be selectively etched is underneath the channels. Figure Ib shows a cross section of the substrate along the line AB in Figure Ia.
A glass substrate 5 was provided with a layer 4 of either silver or aluminium, lOOnm thick, by vacuum evaporation. Capillary channels 2 were created on top of the glass substrate 5. The channels were created by patterning a layer of 50 micrometer thick Laminar™ photoresist 3 using standard lithographic techniques. The channels 2 were created with a range of widths but generally a width of 100-300 micrometers was used. A pattern as shown in Figure Ia was created. Each channel was in contact with a reservoir region 1 and a sump region 6. The channels had a square cross-section and were open at the bottom, so that any liquid in the channel was in direct contact with the metal layer at the bottom of the channel. In order to remove metal from the bottom of the channel, a liquid etchant was introduced into the channel at the reservoir region 1 and was drawn along the channel by capillary forces, so that eventually the entire channel was filled with liquid etchant. For this to occur the contact angle of the liquid etchant on the interior surface of the channel should be as low as possible and generally at least less than 90 degrees. However, the majority of liquid etchants that can be used for removal of metal are aqueous solutions of concentrated acids which due to the high contact angle usually prevents effective filling of the channels by capillary forces. To facilitate the use of this class of etchants, a surfactant was used to lower the contact angle ofthe etchant on the channels. Due to the very low pH of the liquid etchant solution, it was not practical to add the surfactant directly to the etchant. Instead, the channels were first wetted with the surfactant by using a non-aqueous solvent such as ethanol or acetone to carry the surfactant into the channels. In this example OHn 1OG was used as the surfactant to increase the wettability of the channels but other surfactants could be used for this purpose. A droplet of the surfactant solution was deposited in a reservoir region by pipette and was drawn along the channels by capillary action leaving a thin layer of surfactant on the surface of the channels after evaporation of the solvent.
After the channels were made more wettable by coating with surfactant, approximately 1 microlitre of a concentrated acid etch, such as hydrochloric acid (HCl), nitric acid (HNO3), sulphuric acid (H2SO4), chromic acid (K2Cr2O7), and phosphoric acid (H3PO4), was deposited into one of the reservoir regions 1 and was drawn along the channels by capillary action. The circulation of fresh etchant at the metal surface was ensured due to the rolling motion present at the advancing contact line of the liquid etchant. After the channels were filled with etchant a delay of several seconds or minutes was allowed, depending upon the etchant being used, before the etchant was removed from the channels by application of suction or a capillary wicking material to the sump region 6. A further processing step of flushing the channel with a low surface tension solvent was performed to prevent further etching of the metal at the base of the channel. A low surface tension solvent such as ethanol or isopropylalchohol, was applied to the reservoir region 1 and subsequently removed from sump region 6 after filling of the channels. After the etching process, complete removal of the metal from the channel region was achieved, as verified by measurement of the electrical conductivity between the inside and outside of the circular channel structure in Figure 1. Example 2 - Etching of conformal metal films in capillary channels
Figure 2a shows another embodiment in which the substrate 25 already has a series of capillary channels 22 patterned on the surface with a conformal metal coating 24 on top of the channels. The channels may be formed using photo-resist 23, as in the previous example, or may be formed by embossing, moulding or micro-replication. In this case the channel defines where the metal is to be removed since the etchant is confined within the channel. Figure 2b shows a cross-section through the substrate along the line AB
In this example the channels were either formed from photo-resist patterned on a glass substrate, having a square cross section as in Figure 2a, or were created by microreplication in a plastic or polymeric substrate resulting in channels with a 'V shaped cross section as shown in Figure 3a.
A sample of polycarbonate with 'V shaped channels coated with a layer of either silver or aluminium approximately lOOnm thick was selectively etched using various concentrated acid solutions: hydrochloric acid (HCl), nitric acid (FTNO3), sulphuric acid (H2SO4), chromic acid (K2Cr2O7), and phosphoric acid (H3PO4). In order to use these liquids as etchants, the surface energy of the channels was increased by first wicking surfactant 1OG along the channels using a low surface tension liquid such as ethanol or acetone. Next approximately 1 microlitre of the etchant liquid was placed into the reservoir region 26 of the channel to be etched, and the liquid was drawn along the channel by capillary action. After the channels were filled with etchant a delay of several seconds or minutes was allowed, depending upon the etchant being used, before the etchant was removed from the channels by application of suction or a capillary wicking material to the channel. If required, a further processing step of flushing the channel with solvent could be performed to prevent further etching of the metal at the base of the channel. A solvent was applied to one end of the channel and subsequently removed from the other end of the channel. The example in Figure 2a has reservoir regions 26 and sump regions 21 to aid the deposition and removal of the etchant liquid from the channels. If the purpose of the etching step is just to prevent electrical conduction along the metallic layer within the channel, then the rinse steps can be neglected, since although metal is re-deposited within the channels, the re-deposited layer is not continuous or uniform so the electrical conductivity is greatly reduced.
Example 3. - Etching of inorganic layers beneath channels Referring to Figure 1 again, capillary channels were created on top of a glass substrate 5 with an inorganic layer 4 of either zinc oxide or indium tin oxide (ITO) of order lOOnm thick , by patterning a layer of photoresist 50 micrometer thick 3 using standard lithographic techniques. Channels 2 were created with a range of widths but generally a width of 100-300 micrometers was used. A pattern as shown in Figure 1 was created. The channels had a square cross-section and were open at the bottom, so that any liquid in the channel was in direct contact with the inorganic layer at the bottom of the channel. In order to remove the inorganic layer from the bottom of the channel, a liquid etchant was introduced into the reservoir region 1 and was drawn along the channel by capillary forces, so that eventually the entire channel was filled with liquid etchant. For this to occur the contact angle of the liquid etchant on the interior surface of the channel should be as low as possible and generally at least less than 90 degrees. However, the majority of liquid etchants that can be used for removal of inorganic oxide materials are aqueous solutions of concentrated acids, which due to the high contact angle, usually prevents effective filling of the channels by capillary forces. A surfactant flush was used to lower the contact angle of the etchant on the channels as described in example 1. Due to the very high pH of the liquid etchant solution, it was not practical to add the surfactant directly to the etchant. To facilitate the use of this class of etchants, a surfactant was used to pre- wett the channels by using a non-aqueous solvent such as ethanol or acetone to carry the surfactant into the channels. In this case OHn 1 OG was used as the surfactant to increase the wettability of the channels but other surfactants could be used for this purpose.
After the channels were made more wettable by coating with surfactant, approximately 1 microlitre of a concentrated acid etch from the following list were deposited into the reservoir regions 1 of the various patterned areas and was driven along the channels by capillary action: oxalic acid (C2H2O4), acetic acid (CH3COOH), hydrochloric acid (HCl), nitric acid (HNO3), sulphuric acid (H2SO4), chromic acid (K2Cr2O7) and phosphoric acid (H3PO4). The circulation of fresh etchant at the surface of the inorganic material was ensured due to the rolling motion present at the advancing contact line of the liquid etchant. After the channels were filled with etchant, a delay of several seconds or minutes was allowed, depending upon the etchant being used, before the etchant was removed from the channels by application of suction or a capillary wicking material to in the sump region 6. If required, a further processing step of flushing the channel with solvent can be performed to prevent further etching of the metal at the base of the channel. A solvent was applied to the reservoir region 1 and subsequently removed from the sump region 6 after filling of the channels.
In this example the channels were either formed from photo-resist patterned on a glass or flexible polymer substrate, having a square cross section or were created by microreplication in a plastic or polymeric substrate resulting in channels with a 'V shaped cross section.
Example 4. - Etching of conformal inorganic layers beneath channels.
Referring to Figure 3, a sample of polycarbonate 35 with 'V shaped channels 32 with a coating of either zinc oxide or indium tin oxide 34 which was approximately lOOnm thick was selectively etched using a concentrated acid solution from the list: oxalic acid (C2H2O4), acetic acid (CH3COOH), hydrochloric acid (HCl), nitric acid (HNO3), sulphuric acid (H2SO4), chromic acid (K2Cr2O7), and phosphoric acid (H3PO4). In order to use these etchant liquids, the surface energy of the channels was increased by first wicking surfactant 1OG along the channels using a low surface tension liquid such as ethanol or acetone. Next approximately 1 microlitre of the etchant liquid was placed into the channel to be etched and the liquid was driven along the channel by capillary action. After the channels were filled with etchant, a delay of several seconds or minutes was allowed, depending upon the etchant being used, before the etchant was removed from the channels by application of suction or a capillary wicking material to the channel. If required, a further processing step of flushing the channel with solvent could be performed to prevent further etching of the metal at the base of the channel. A solvent was applied to one end of the channel and subsequently removed from the other end of the channel.
Example 5 - Etching of polymer layers Referring to Figure 1 , capillary channels were created on top of a glass substrate 5 with a layer 4 of conductive polythiophene polymer (Baytron F CPP105M, Bayer) of order lOOnm thick, by patterning a layer of 50 micrometer thick photoresist 3 using standard lithographic techniques. A pattern as shown in Figure 1 was created. The channels had a square cross-section and were open at the bottom, so that any liquid in the channel was in direct contact with the polymer layer at the bottom of the channel. In order to remove the polymer layer from the bottom of the channel, a liquid etchant was introduced into the reservoir region 1 and was drawn along the channel by capillary forces, so that eventually the entire channel was filled with liquid etchant. After the channels were filled with etchant, a delay of several seconds or minutes was allowed, depending upon the etchant being used, before the etchant was removed from the channels by application of suction or a capillary wicking material in the sump region 6. If required, a further processing step of flushing the channel with another solvent could be performed to prevent further etching of the polymer at the base of the channel.
In this example the channels were either formed from photo-resist patterned on a glass or flexible polymer substrate, having a square cross section or were created by microreplication in a plastic or polymeric substrate resulting in channels with a 'V shaped cross section. Hexan-1-ol, Octan-1-ol and other longer chain alcohols were found to be most effective etchants for the polythiophene layer, since they had sufficiently low surface tension to promote wicking and were a suitable solvent for the polymer layer. Before etching, the polymer layer had a resistance of order 0.2 Mega Ohms when measured between the inside and outside of the circular channels 2 in Figure 1. After etching the resistance rose by an order of magnitude to more than 2 Mega Ohms.

Claims

1. A method of selectively removing material from a surface, the method comprising the steps of: c) providing the surface with at least one open channel, wherein the channel defines the region where material is to be removed d) depositing a flowable etchant adjacent to the said at least one open channel, such that the etchant is drawn into the channel by capillary forces wherein material is transferred from the surface into the etchant.
2. A method as claimed in claim 1 wherein material at the surface is solubilised by the etchant.
3. A method as claimed in 2 wherein material is further removed from the surface by a physical process.
4. A method as claimed in claim 1 wherein etchant is removed from the channel by continuous flow.
5. A method as claimed in claim 4 wherein the channel is provided with a sump region.
6. A method as claimed in claim 1 wherein the channel is provided with a reservoir region.
7. A method as claimed in claim 1 wherein etchant is removed from the channel by suction.
8. A method as claimed in claim 1 wherein etchant is removed by transfer into an absorbent material in contact with the channel.
9. A method as claimed in claim 1 wherein the transferred material is redeposited on the surface, the redeposited material having different bulk properties to the material prior to its contact with the etchant.
10. A method as claimed in claim 1 wherein prior to depositing the etchant, the channel is treated to modify its surface properties.
1 1. A method as claimed in claim 1 wherein the channel comprises regions with differing etch rates.
12. A method as claimed in claim 1 wherein the etchant is deposited by an additive printing method.
13. A method as claimed in claim 1 wherein the etchant is made flowable by the application of heat
14. A patterning substrate for patterning etchable material having a surface provided with at least one channel and at least one sump region and at least one reservoir region wherein the reservoir region is located in the surface adjacent to said channel such that their relative positioning would result in flowable etchant in the reservoir region being directed along said channel by capillary forces and wherein the sump region is located in the surface adjacent to said channel such that their relative positioning would result in flowable etchant in the channel being directed into the sump region
PCT/US2009/002992 2008-05-23 2009-05-14 Method of etching WO2009142704A1 (en)

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GB0809396.5 2008-05-23

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10214444B2 (en) 2013-06-07 2019-02-26 Commissariat à l'énergie atomique et aux énergies alternatives Method for treating a surface and device implemented

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
STARK R W ET AL: "Microfluidic etching driven by capillary forces for rapid prototyping of gold structures", MICROELECTRONIC ENGINEERING, ELSEVIER PUBLISHERS BV., AMSTERDAM, NL, vol. 67-68, 1 June 2003 (2003-06-01), pages 229 - 236, XP004428874, ISSN: 0167-9317 *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10214444B2 (en) 2013-06-07 2019-02-26 Commissariat à l'énergie atomique et aux énergies alternatives Method for treating a surface and device implemented

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