GB2395357A

GB2395357A - Plasma etching fluorinated polymer substrates

Info

Publication number: GB2395357A
Application number: GB0226688A
Authority: GB
Inventors: David Anthony Barrow; Nicola Harries; Tyrone Gwyn Jones; Kostas Bouris
Original assignee: University College Cardiff Consultants Ltd; Cardiff University
Current assignee: University College Cardiff Consultants Ltd; Cardiff University
Priority date: 2002-11-14
Filing date: 2002-11-14
Publication date: 2004-05-19
Also published as: AU2003282248A1; WO2004044655A1; GB0226688D0

Abstract

A method for the selective bulk-machining of a fluorinated polymer substrate: the method comprises the steps of dry etching the substrate (1) using a reactive ion plasma (8). The method enables the precision machining of miniature articles, for example microfluidic devices.

Description

l Method for the Bulk Machining of Fluoropolyner Substrates llackgrounl

The nniniaturisalion of, and creation of new, three-dimensional objects (e.g. sensors, actuators, ducts for fluids, waveguides for photons) has required new techniques for the precision 5 machining of naterials in three dimensional space. Precision and subtractive machining need both be measured in micrometers and nanomelres. An example is the creation ol' devices with ducts and containers for fluids where such ducts may measure typically (but not exclusively) lOOmicrons in linear dinncusion (depth, width, diameter) but may range from -1OOnanoinetrcs lo sevarl hundred micrometres. Whilst niniaturised devices may be manufactured from a diversity lO of materials such as silicon, ceramics, glass and metals, polymers, are frequently considered to hold an advantage of low unit cost when manufactured in large quantities. Polymer substrates in which micrometer and nanomctre scale features or devices are machined may include a wide range ol' materials, including polycarbonale, polysulphone, polyolefin, polyelherelherkelone, polyurethane, polymethyhnethacrylate and many others. Such materials have a wide 15 applicability to numerous industrial uses. In microlluidic applications, particularly where

aggressive liquids or other conditions are required, materials must be relatively inert to chemical degradation and stable at elevated temperatures. This leads to a preference for flouropolymer and perDuoropolymer materials based upon polylelrafluoroethylene (PTF13), such as 'l'eflon and CYTOP'M.

20 'l'he construction of devices with micro- and nano-metre feature size, or of micro- and nano-

metre absolute size, by the bulk machining of polymer (particularly fluoropolyners) substrate materials, (as compared with surface micromachining of thin applied fihns) may be undertaken by several techniques. These include embossing, molding, ion-beam milling, laser ablation and inechanical milling. Despite iovalions in these techniques all the requirements of industry 25 have not been met. For instance, the tooling for injection molding is expensive and methodologies l'or precision machining of the tool inserts lacks resolution al the submicron scale. With hot embossing, tools are again expensive and tineconsuming to create, and frequently too delicate to last for many hundreds of thousands of impressions. Some polymers such as PTFE ares not thermoplastic and, therefore, not amenable to microembossing or 30 micromolding. Laser ablation is a useful tool for machining of polymers but for ultraviolet transparent polymers such as PTFE, very short wavelength lasers frequently have to be used which require very expensive optics and atmospheric control, arc complicated in operation and still do not produce surface finish requirements as required by many industrial applications such

as in fluidic and optical devices. Reactive ion etching is a tecimique commonly used in the microelectronics industry for patterning and removing thin films applied to other substrates.

Such fihns Night include dielectric films to coat and insulate electrical conductors, or photoresists as part of a lithography process. Glow discharge plasmas have also been utilised for 5 cleaning a variety of surfaces and the modification of surl'aces to alter the surface states.

However, whilst reactive etching has been used extensively for bulk machining of silicon and other semiconductor materials, glass, quartz and ceramics, it has not been further developed to machine substantially into the bulk substrate of polymers to create structures with a substantial Taxis dimensions, particularly for microfluidic devices. It is a purpose ol' the invention disclosed 10 here lo provide a methodology for the selective subsiraelive machining ol' bulk fluoropolymer substrates, which provides particular production advantages for prototyping and massfabrication of nuoropolymer based rnicrodevices, particularly for microDuidic devices and components.

Description of Invention

15 In accordance with a first aspect, a method is described for the selective, bulk-machining of fluoropolymer substrates with micron and submicron precision. Most prel'erably, fluoropolymers include but not limited to PTFE, Teflon_, Teflon AF, Teflon NXT, Teflon G. Tenon PFA, Teflon PEA LIP Plus, Dyneon, CYTOP'rM. The technique employs reactive ion etching with plasmas of oxygen, oxygen-argon, oxygen-helium, oxygen+ carbon tetrafluoride 20 (CF4), oxygen+sulphur hexafluoride (SF6) and oxygen-helium-argon which removes the polymer by means of energetic ions and molecules. The tecluique is distinguished from reactive ion etching of polymer films applied to another substrate (such as a photoresist or laminate on a silicon of glass wafer) in that it is the actual bulk subsirale that is subtraetively machined. The features may measure less than l()()nm in linear dimension or be as large as 5()0 micrometres 'in 25 linear dimension. In particular, reactive ion-etched removal of the substrate may be undertaken to machine part the way through the bulk substrate material to form ducts and cavities the base of which is made from the same substrate rnalerial. This contrasts with typical use of reactive ion etching that is frequently utlised to machine through a layer applied to another. Using this method small feature-sizes (<l00nanometeres) and anisotropic, high-aspect-ratio features (20:1, 30 depth to height) can be obtained.

Reactive ion etching is a parallel process in that all areas of substrate exposed to the plasma are etched at the same time which renders it suitable to manufacturing of substrates suitable for a wide diversity of microscale devices including those intended for microfluidics, microchemistry and other applications where a relatively large substrate format is required. Advantageously, the

reactive ion etchings process lends itself to both rapid protyping, of one-off substrates, to small-

baLch production and further to mass-production by virtue of available equipment comnonly employed for silicon and silica micromachining. Automated cassette-loadhg systems with lary,e wafers facilitate highvolunne production with the advantage that complex nanoscale features 5 could be incorporated which is not easily achieved with volume micromanufacturing techniques such as injection micromolding. Since reactive ion etching may be used as both a prototyping and massmanufacturing process it provides for a seamless transfer of processing technology to different scales of production. This holds an advantage over embossing and injection molding where separate tools have first to be constructed which is both time-consuming and expensive.

10 Etched geometries in substrates may be enabled by protection of the substrate using simple lithographic patterning of an evaporated or sputtered metal mask Lprel'erably Hi or Cr. less preferably Al, Ni, (and alloys of), silicon, silicon dioxide]. Where metal masks are utilised these may optionally be left on the substrate post-etching and serve other purpose. For example, in nicroscale devices and systems which incorporate ducts for the transmission or containment of 15 fluids, the mask may be retained to provide a selective light block (through the un- etched substrate) for through-transmissive illumination of fluid flows in ducts where fluid properties (e.',. but not limited to, velocity, particle morphology, density, refractive index, colour spectrum) require to be detccled/imaged or where fluids and or suspended particulatcs require illumination for a given reaction process. Equally, where Al and Ti mask films are utilised these may, 20 optionally, be left on the surl'ace of the substrate facilitating the direct adhesion to (i) a bulk fluoropolymer or a [luoropolymer coated cover layer OR (ii) to an interlayer film (hcluding Teflon Ad, 'I'eflon NXrl', Teflon G. Teflon l'F'A, 'Teflon l'l-'A HI' Plus, Dyneon, and CY'1'OP Buoropolymer) that in turn allows joining lo a bulk nuoropolymer or a iluoropolymer coated cover layer. During assembly of substrate and any coverlayer(s) the thin metal interlayer fihn 25 may be optionally heated by inductive coupling, thus causing the temporary and localised nellhg of any thermoplastic material and causing a Buid tiy,ht seal. Equally, the metallic Ulna may be processed further to produce one or snore electrical conductors or an electrical ground plane. These examples serve Lo illustrate how the mask layer may provide both the means of selectively patterning the etch process and also other purposes beyond the etch process. Those 30 pracLised in the art will recognise from benefit of this disclosure that several other posL-

processing l'unctionalities may be made from the metallic mask film.

In accordance with another aspect the etch rate of bulk machining of fluoropolymer substrates using reactive ion etching may be accelerated by cooling the plasma and/or substrate which leads

to very high removal etch-rates (>7un/min). Cryogenic cooling may be enabled to reduce temperatures to <0 C or more preferably to <-100 C. Manut:acturing costs of components made by etching are determined, hi part, by processing thee. Accelerated etching thnes by use of cooled plasma anchor substrates allows faster processing operation to be done with the result of 5 higher unit lhroughpul. Unit cost of manufacture are thus reduced. Prolonged exposure to reactive plasmas may cause physical and chemical alterations to the bulk of thermally sensitive substrate materials. The accelerated plasnna processing time by use of cooling plasma and/or substrate lessens any such effects.

10 In accordance with another aspect the etch rate of bulk machining of fluoropolymers using reactive ion etching may be accelerated by use of an inductively coupled plasma whereupon, because of the increased plasma density in the vicinity of the bulk substrate, even higher removal elchrates (>15 um/min) are obtained.

15 In accordance with another aspect the spatially variable bulk machining of polymers using reactive ion etching may be enabled by protecting the substrate with a shadow mask that is placed over the substrate and caused to be in intimate contact with it. The shadow mask may be constructed from silicon, silicon nitride, glass, ceramic silicon dioxide and metals. One embodiment of the shadow snack comprises a silicon wafer over which a silicon nitride film has 20 been deposited and selectively machined to provide openings to the underlying substrate. The silicon substrate may then be removably etched over a much larger surface area but with areas of bulk silicon lelt to provide mechanical strength to the shadow mask. When such a constructed shadow mask is applied to the polymer substrate to selectively protect it from etching by the reactive plasma the sub-micron thinness of the silicon nitride membrane provides a very short 25 and favourable path length access of the plasma to the substrate. It will be clear to those practiced in the art, given the advantage of this disclosure, that suitable shadow masks may be

created by several other means such as the laser ablation of openings in a metal foil.

In accordance with another aspect the spatially variable bulk machining of lluoropolymers using 30 reactive ion etching may be enabled by greyscale lithographic definition of a positive pholoresist mask layer, the pattern of which is subsequently transferred to the fluoropolymer substrate by the reactive ion-elch process. I he ratio of the etch rate of the photoresist mask layer to that of the fluoropolymer substrate is used to calculate the 3-dimensional geometric profile of the polymer mask layer that is required to achieve a given geometric etched pattern in the underlying bulk

substrate. From these geometries the pattern of the greyscale lithographic mask is derived. Using this method described it is necessary to arrange that the etch process is stopped at the correct time such that the parts of the bulk substrate which are not required to be etched remain in such a state. 'l'o assist this requirement an additional process in the method described here involves 5 the addition of a patterned metal mask layer underlying the positive photoresist mask layer. This additional metal mask layer may serve the purpose of an etch stop, due to its comparatively high etch resistance to the plasma.

Descriptions of the Figures

ID IlGSlA-H shows a sequence of process operations by which a reactive plasma is used to etch substantially lhree-diinensional geometrical features within the bulk material of a substrate.

Substrate 1 is coated with a metallic film 2 by evaporation or sputtering (FIG I B). I'hotoresist 3 is coated over the metallic film 2 (FIGIC) and exposed to light source 5 through lithographic mask 4 (FIG1D). The pholoresist 3 is processed after exposure to light by baking and developing with IS appropriate developer compounds. The photoresist layer is rinsed leaving windows 6 within it allowing access to the metallic film 2 beneath (FlGIE:). The substrate I with metallic film 2 and patterned photoresist 3 is exposed to a bath of wet chemical etch so that the exposed metallic film is etched away revealing areas of substrate I (FIGIF). After subsequent rinsing and drying the coated substrate is placed within a chamber in which is caused to occur a reactive plasma 8 20 (I;lGlG). Alder a period of thee, usually at least several minutes, frequently up to one hour or more, the substrate is withdrawn from the reactive plasma environment and the photoresist removed with appropriate solvents. Etched feature 9 remains as evidence of the subtractive bulk etching process in substrate I (FIGIH).

25 FIGS2A-C shows another sequence of process operations by which a reactive plasma is used to etch substantially thrcc-dimensional geometrical features within the bulk material of a substrate.

Substrate l is provided with a shadow mask ID that is placed in intonate juxtaposition with its surface (FIG2B). This intimate juxtaposition may be enabled by means of mechanical, adhesive, or vacuum clamping, or by the application of a weight. Optional corresponding registration 3() features 14 on the substrate and/or shadow mask may assist with alignment. The substrate and shadow mask are exposed lo reactive plasma 8 that may be cooled and which substrate may also be cooled during the exposure. After a period of time, usually several minutes, more t'requently up to one hour or more, the etched substrate is removed frown the plasma. The shadow mask is also removed revealing a substrate that incorporates etched features 9 etched in a spatially

variable mariner the pattern of which corresponds to the pattern of openings in the shadow mask.

The plasma 8 may also be intensified in the region close to the substrate by inductive coupling of the plasma.

5 FlGS3A-I shows another scqucncc of process operations by which a reactive plasma is used to etch substantially three-di''lensional geometrical Features within the bulk material of a substrate.

Substrate 1 is coated with a metallic film 2 by evaporation or sputtering (FIG3B). Photoresist 3 is coated over the metallic John 2 (1;IG3C) and exposed to light source 5 through lithographic mask 4 (FlG3C). The photoresist 3 is processed after exposure to light by baking and developing with 10 appropriate developer compounds. The photoresist layer is rinsed leaving windows 6 within it allowing access to the metallic film 2 beneath (FlG3D). 'lathe substrate with metallic film and patterned photoresist is exposed to a bath of wet chemical etch so that the exposed metallic film is etched leaving openings 7 that expose areas of substrate l (FlG3E). After subsequent rinsing and drying the coated substrate is further coated with a Slicker positive photoresist layer 10 by 15 spincoaling, spray-coating or casting (FIG3F). The photoresist layer lO is exposed lo light through a grayscale photolithographic mask l l (AGOG). Mask l l allows the transmission of light in a spatially variable manner such that light may be variably transmitted at any percentages from zero to one hundred percent in increments of at least 10% percent, usually at least 0.39% which is compatible with a 256 tonal grayscale range. After exposure the resist is processed by 20 baking and developing with appropriate developer and rinsing compounds. This development and rinsing process leaves geometrical features 12 within the photoresist 10 which are geometrically variable in the z-axis dimension. 'l'he substrate with structured layers of metallic mask 2 and photoresist 10, is placed within a chamber in which is caused lo occur a reactive plasma 8 (1F'lG311). After a period of time, usually at least several minutes but also extending to 25 an hour or more, the photoresist is etched away. Simultaneously the substrate is etched leaving etched features 13 (FIG31) of a morphology that reflects the morphology of the geometrical features 12 within the structured photoresist 10. The metallic mask 2 provides a convenient etch stop such that the reactive plasma does not continue to etch certain parts of the substrate when the structured photoresist 10 is completely removed. The substrate is withdrawn from the 30 reactive plasma environment and the metallic mask layer 2 optionally removed with appropriate applied solutions. The process described here may be applied without the use of the metallic mask 2 so that there is no etch stop layer.

Claims

1. A method for the selective bulk-machining of a fluorinated polymer substrate, comprising the step of dry 5 etching the substrate using a reactive ion plasma.

2. A method as claimed in claim 1, comprising the step of cooling the plasma and/or substrate.

10

3. A method as claimed in claim 2, in which the plasma and/or substrate are cooled to below 0 C.

4. A method as claimed in claim 2, in which the plasma and/or substrate are cooled to below -100 C.

5. A method as claimed in any preceding claim, in which an inductively coupled plasma is used.

6. A method as claimed in any preceding claim, in which 20 the substrate is provided, on a surface or surfaces to be etched, with an evaporated or sputtered metallic or non-

metallic mask.

7. A method as claimed in any one of claims 1 to 5, in 25 which a shadow mask is disposed over and in intimate contact with a surface or surfaces of the substrate to be etched.

8. A method as claimed in any one of claims 1 to 5, in 30 which the substrate is provided, on a surface or surfaces to be etched, with a polymer photoresist mask layer

providing a greyscale lithographic pattern which is transferred into the substrate by the etching step.

9. A method as claimed in claim 8, in which the substrate s is additionally provided with a metallic or non-metallic mask layer below the polymer mask layer.

10. A method as claimed in any preceding claim, in which the material of said substrate comprises a fluoropolymer, a 10 perfluoropolymer, or a fluoro-elastomer.

11. A method as claimed in any preceding claim, in which the reactive ion plasma comprises a plasma of oxygen, oxygen-argon, oxygen-helium, oxygen+ carbon tetrafluoride, 15 oxygen+sulphur tetrafluoride or oxygen+helium+ argon.

12. A method as claimed in any preceding claim, in which the etching is effected part the way through the substrate to form one or more ducts, cavities or lamellae, or fin 20 like, pillared or other three-dimensional morphologies, in which all etched surfaces thereof are of the fluorinated polymer substrate.