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A method for local application of thin organic adhesive films on micropatterned structures
MATERIALS SCIENCE & ENGINEERING ELSEVIER
Materials Science and EngineeringC 5 (i998) 227-231
C
A method for local application of thin organic adhesive films on micropatterned structures H. Dreuth, C. Heiden Institute of Applied Physics, Universityof Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany Received i0 December 1996
Abstract
The application of adhesive films can be an important step in the manufacturing process of micromechanical structures if precise control of very small volumes of the adhesive over small contact areas is guaranteed. Residues of adhesives outside the joint area are to be avoided. Conventional microdispensers allow the handling of volumes down to about I nl. Recent developments employ techniques used in ink jet printers and are capable of delivering single drops of adhesive with volumes as low as 5 pl. Some micromechanicaljoints may require volumes even lower (an adhesive joint of 200 nm thickness over an area of i0 b~mX 10 b~m requires for instance a volume of 0.02 pl). The method developed for this purpose involves the deposition of a thin organic adhesive fiIm by a spin coating process and the controlled transfer to (and only to) the intended locations on the micromechanical structure. One beneft of this method is increased design flexibility due to extension of the range of combinations of conventional thin film technology with other materials such as polymer films. The method was tested by preparing small cavities and other simple three-dimensional microstructures where it was mandatory to prevent the adhesive filling up the voids between the contact areas. Examples of composite structures made of 0.6 ~m thick PET films glued to micropattemed structures on silicon substrates are presented. © 1998 Elsevier Science S.A. Keywords: Adhesion; Organic films; Polymer films; Silicon
1. I n t r o d u c t i o n In the production of micromechanical devices adhesives are often employed for mounting purposes only but not for the manufacture of the micromechanical structures themselves. One main reason is that assembling micromechanical parts with adhesives requires precise appIication of very small volumes of the glue to exactly defined areas of the sample surface. Commonly used methods for applying small volumes of adhesives are applied for surface mounting electronic parts and allow doses not much less than 1 nl. Novel methods using piezoceramic actuators coinparable with those in ink jet printers have been shown to handle volumes down to 5 pl [ 1]. Adhesive films for microsystem assembly may require even lower volumes and a thickness comparable with the respective dimensions in thin film technology which can easily be in the range of 100 nm. Here we report on micromechanicaI structures manufactured from thin polymer foils and silicon microstructures with a novel method for micromechanical adhesive joints. The requirements for these adhesive joints and the adhesive used are as follows. 0928-4931/98/$19.00 © 1998 Elsevier Science S.A. AII rights reserved PIIS0928-4931 ( 97)00048-9
• The joint strength must be satisfactory for the application. • In order to glue micromechanical structures produced using thin film processes, a thickness of the adhesive layer of less than a micrometer is to be achieved. • The method of applying the adhesive should allow control of the areas to be coated on a micrometer scale. Volumes to be applied onto a given contact area thus can be much less than a picoliter. The process employed to fabricate adhesive joints according to these specifications consists of two steps. l. A spin coating process is used to obtain a thin film of the adhesive on a substrate. The thickness of the layer can be controlled by adjusting the initial viscosity of the adhesive and the spin speed. 2. A stamp printing transfer of the adhesive layer onto the elevated features of the structure is performed. Here, the structure to be coated plays the role of the stamp itself. The stamp printing of the adhesive poses an additional restriction on the choice of glue: its theological behavior is required not to drag fathoms when the stamp is removed from the spin coated surface.
228
H. Dreuth, C. [-leiden / Ma;erials Science and Engineering C 5 (t998) 227-231
2. Experimental details
state and therefore no heat cure of the joint is required, they remain sticky at room temperature for days (the glass temperature of the polymer is about - 5 5 °C, see [3], p. 230). This adhesive also can be arbitrarily diluted in various solvents to obtain low viscosities as required for the spin process (the viscosity has to be in the order of 1 mPa s). The adhesion mechanism between this adhesive and the corresponding adherends involves only physical interactions ( dipol and dispersive forces). One drawback of this adhesive is its smaller adhesiveness compared with chemical reactive adhesives. Whether the adhesiveness is sufficient for the intended use is therefore to be determined. In our case where parts of thin polymer foil are involved, the strength of the adhesive joint was acceptable since the very thin polymer foil itself can onty withstand small absolute forces.
Owing to its favorable electrostatic properties and because of the availability of high quality thin films, polyethylene terephthalate (PET) was chosen as polymer. Thin membranes of PET (down to a thickness of 0.6 b~m) were to be bonded adhesively to microstructures on silicon wafers.
2.1, The selection of the adhesive In order to create a thin adhesive film on a flat substrate by a spin process, suitable viscosity of the adhesive is essential. Therefore the main criterion for the choice of adhesive was that the viscosity of this adhesive mass can easily be tuned by adjusting the fraction of diluting solvent. Second, heat curing of a reactive adhesive was not acceptable because of the glass temperature of PET which is only 65-68 °C [2] (see [ 3 ], p. 287). Third, the thin adhesive layer prepared by spinning had to remain sticky at least for the time necessary to complete the adhesive joint and should not for example harden prematurely owing to its small volume and large surface. Also diffusion adhesion (see [4], p. 428ff; [5], p. 341 ) is not an appropriate method for bonding PET because PET exhibits good chemical resistance (see [5], p. 414). The adhesive chosen was polybutyl acrylate diluted in aceton/benzine (manufactured by Beiersdorf AG, Hamburg) ; it is usually used for the production of adhesive tape. Polybutyl acrylate fulfils the above requirements. While its polymer molecules are already present in their final chemical
2.2. Process description The process illustrated schematically in Fig. 1 involves several steps. The microstructures onto which the PET foil was to be glued were manufactured as follows. First a photoresist mask was structured on a silicon substrate by standard lithographical methods, Second a layer of tungsten titanium (thickness about 600 nm) was sputtered onto the sample, Then a lift-off process in acetone completed the structures, To avoid uncontrolled deposition of glue between the microstmctures, the thickness of the adhesive layer should be less than the height of the microstucture, In order to achieve layers less than 1 p,m thick by spinning the adhesive onto a
substrate
h\ structure
adhesive
~
r,,
..~ubs,tra-tez
t/
I. Substrates brought into contact
PETfoil
adhesive
? II. Pressure applied TFI,Substrates separated
~ f r - - v ~-f/
IV:.PET foil joined to substrate V, Completed Structure Fig. 1. Schematicdiagramof the process.
H. Dreuth, C. Heiden/Materials Science and Engineering C 5 (1998) 227-231
substrate, it was necess~y to dilute the original adhesive mass whose solid fraction amounted to 44%. Tile solvents tested were acetone and ethyl acetate. With these solvents the original adhesive mass was diluted by about 1 part of the adhesive mass to 30 parts of the respective solvent. This solution then was spun onto the substrates at 1000-5000 rev min -l. The thin adhesive layer was then transfered onto the elevated microstructures by pressing the two corresponding substrates together. The pressure is not applied primarily to increase the effective area of every adhesive joint itself or to ensure a uniforin thickness of the adhesive joint, but to achieve close contact between the wafer with the thin adhesive layer and all the elevated structures of the sample. The amount of pressure has to be adjusted such that first it is high enough that all elevated structures are in contact with the adhesive layer, and second the substrates are not bent too much. Bending the substrate could otherwise cause the adhesive layer to touch parts below the elevated structures or adhesive to be squeezed out of the joint areas. To apply the appropriate pressure a vice was used. The jaws that press onto the substrates were attached with an additional thin layer of rubber to even out mechanical inaccuracies of the arrangement. The force exerted on the samples could be adjusted by applying the appropriate torque to the vice. The transfer of the adhesive layer to the elevated structures was then completed by separating the two substrates. To attach the thin polymer foil to the sample the foil was carried by a frame. The two pieces were then simply put together to form the adhesive bond. To prevent the foil from being stretched between the contours of the microstructure, the pressure was not applied mechanically onto the foil. Instead a stream of nitrogen was applied to achieve contact between the foil and the adhesive. Since the adhesive only resided on the elevated structures, the foil only adhered there, leaving cavities between these structures. Because of the dimensions of the structures and adhesive layers, all these processes were performed in a cleanroom environment.
2.3. Process parameters Various parameters can influence the quality of the process described above. Most important are the following: • the type of substrate for spin coating of the adhesive, • the spin speed (which determines the thickness of the adhesive layer), • the solvent used for diluting the adhesive, • the stamp printing pressure, • the mechanical arrangement for the stamp printing of the adhesive, • the geometry of the microstructures to be coated. For the tests simple geometries such as stripe patterns were used for the microstructures. They are characterized by their aspect ratio which is defined as the ratio of their height and the distance between them.
229
As substrates for the adhesive usually silicon wafers were used. For comparison also polished PTFE substrates were used to evaluate whether a material with very low surface energy would facilitate the transfer of the adhesive layer.
3. Results
3.1. Properties of the spun adhesive layers Polybutyl acrylate showed different properties when diluted either in acetone or in ethyl acetate. While a solution in acetone did not yield as homogeneous adhesive layers as a solution in ethyl acetate, the latter showed more tendency to draw threads when the substrates are separated. With this solution the thickness of the adhesive film could be varied in the range from about 200 nm down to less then 100 nm with satisfactory homogeneity. The thickness was estimated through the film interference colors. The substrates the adhesive was spun onto have to be as smooth as possible in order to allow a successful transfer of the adhesive layer to the microstructures. Mostly polished silicon wafers were used for this purpose. A test with polished polytetrafluoroethylene (PTFE) substrates showed that the adhesive layer could be transfered to the microstructures more completely than if it were transfered from a silicon substrate. This has been attributed to the very low surface energy of PTFE which is only 18.5 mJ m -2 (see [4], p. 235). Although acetone and ethyl acetate both exhibit higher surface energies of about 24 mJ m - 2 at room temperature (see [6], p. F-33), spin coating of the whole adhesive solution could be performed successfully. Nevertheless, the use of PTFE substrates was discontinued because the relative softness of PTFE compared with silicon made the stamp printing transfer of the adhesive onto the microstructures more sensitive to variations in parameters such as the pressure applied during the transfer, leading to a greater chance that adhesive would be spilled in between the structures. The time elapsed between the spin coating process and the stamp printing of the adhesive was varied between about 5 rain and 1 day. No significant effect of these time intervals on the quality of the process was observed. This corresponds to the fact that the polybutyl acrylate adhesive layer retains its stickyness for even longer periods of time.
3.2. Importa~zt parameters of the stamp printing process To perform a successful transfer of the thin adhesive layer from the substrate to the microstructures two important points had to be taken into account. The most critical parameter was the pressure to be applied. It had to have a suitable value and also needed to be applied uniformly over the whole sample. On the other hand it was also necessary to avoid lateral movements both when the substrates were first put into contact and when they were seperated. Otherwise additional spilling of the adhesive could occur. Appropriate pressures exerted onto
230
~
H. Dreuth, C. Heiden/Materials Science and Engineering C 5 (1998) 227-231
-
: ~. ~ ~
r-~.tl~ll~itt,;{
~i ] : k
~
i°~ ~, ~,
i. . . . . ,
}-
Fig. 2.3 lxm stripes correctlycoated with adhesive. the substrates were found to be on the order of some 1 X 10 6 Pa. It turned out that the mechanical arrangement using a vice for controlling the stamp printing was not sound enough to ensure constant quality of the process. The separating process was performed manually and was also a possible cause of lateral movements when the two substrates were in contact, which causes adhesive to be moved from the elevated structures to locations where it should not be deposited. Fig. 2 shows a sample where the 3 ~m stripes are correctly coated with the adhesive after the stamp printing process. In Fig. 3 another part of the same sample demonstrates that lateral movement during the contact of the substrates can cause parts of the adhesive to be drawn across the sample. 3.3. Influence o f the geometry o f the microstructures
Depending on the aspect ratio of the microstructure two different problems were encountered. First, at high aspect ratios (where the microstructures are located relatively close to each other) the results are especially sensitive even to small amounts of adhesive spilled from those structures between them. This might be tolerable in the second case where the distance between the structures is relatively large (low aspect ratio). Here the chances are that owing to too much pressure during the stamp printing process the two substrates touch
Fig. 4. Completedstructure with high aspect ratio. each other not only at the elevated microstructures but also on some areas in between. The optimal pressure thus also depends on the aspect ratio. The samples used here had aspect ratios between 0.2 (3 Ixm distance between the stripes with thickness of about 0.6 ~Lm) and 0.00057 ( 1 mm distance between structures of 0.57 p~m height). Fig. 4 shows a SEM picture of a sample where the tungsten titanium spacers have a width of 10 p~m. While a 0.9 ~Lm thick PET foil is attached to these spacers the volume between them remains void. This sample was prepared using the techniques described before. Then the silicon wafer was broken and the PET foil torn off at the edge to allow inspection of the cavities. The spatial uniformity of the adhesive transfer on the area of the whole sample depends strongly on the mechanical precision with which the transfer was carried out and also on the other various parameters described above. Sufficient uniformity was achieved on samples where both substrates consisted of whole wafers (of 2 inch diaineter) with low aspect ratio structures. Mechanical inaccuracies of the stamp printing then did not influence the results to the extent that they did on small samples where the uniformity was not satisfactory. Improvements in the mechanical arrangement of the stamp print process are expected to solve this problem.
4. Conclusions
Fig. 3, Adhesive drawn across the 3 gm stripes•
The method described in this paper allows the micromechanical combination of materials which are usually processed with very different methods that cannot be applied equally to both types of material. The use of thin PET foils allowed very thin membranes of relative large area to be attached onto micromechanical structures which would be very difficult if only thin film technology were used. To bond the foil onto the micropatterned structures without filling the area between them, the volumes of the adhesive to be applied had to be lower than could be applied using direct dispensing
H. Dreuth, C. Heiden / Materials Science and Engineering C 5 (1998) 227-231
techniques or combinations of dispensing and then stamp printing the adhesive. A combination of a spin coating process of an adhesive layer together with a stamp printing technique permitted reduction of the volumes applied on a single adhesive joint from the nanoliter range down to well below picoliters. Although this method can be applied to various combinations of materials and possible applications, it must be realized that there are requirements to be met that not every adherend/ adhesive system fulfils. Restrictions here include chemical and physical properties of the adhesive and geometrical properties of the microstructures as well. To enhance the reliability of the stamp printing process, optimization of the mechanical arrangement will be required. Here coating of adhesives was only performed on elevated structures, using these structures directly as a stamp. Applying adhesives onto areas not elevated compared with the surrounding structures in a controlled way would require an
231
additional step where an auxilliary stamp transfers the adhesive onto the designated areas. This stamp must be designed according to the geometry used. Work towards joining adhesively micromechanical structures made of thin polymer foils only is in progress.
References [1] A. Hardwig, M. D6ring, O.-D. Hennemann, Kleben und Dichten, Jahrgang 40, 7-8, 1996, pp. 31-33. [2] R.N. Haward, Colloid Polym. Sci., 258 (1980) 643-662. [3] J.M.G. Cowie. Chemie und Physik der Polymeren, Verlag Chemie OmbH, Weinheiin, 1976. [4] G. Habenicht, Kleben: Grundlagen, Technologie, Anwendungen, Springer, Berlin, 1990. [ 5 ] A.V. Pocius, R.D. Waid, S.R. Hartshorn, in J.D. Minford (ed.), Treatise on Adhesion and Adhesives, VoL 7, Marcel Dekker, New York, 1991, pp. 333-435. [ 6 ] R.C. Weast, M.J. Astle, W.H. Beyer (Eds.), Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1987, 67th edn.
Materials Science and EngineeringC 5 (i998) 227-231
C
A method for local application of thin organic adhesive films on micropatterned structures H. Dreuth, C. Heiden Institute of Applied Physics, Universityof Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany Received i0 December 1996
Abstract
The application of adhesive films can be an important step in the manufacturing process of micromechanical structures if precise control of very small volumes of the adhesive over small contact areas is guaranteed. Residues of adhesives outside the joint area are to be avoided. Conventional microdispensers allow the handling of volumes down to about I nl. Recent developments employ techniques used in ink jet printers and are capable of delivering single drops of adhesive with volumes as low as 5 pl. Some micromechanicaljoints may require volumes even lower (an adhesive joint of 200 nm thickness over an area of i0 b~mX 10 b~m requires for instance a volume of 0.02 pl). The method developed for this purpose involves the deposition of a thin organic adhesive fiIm by a spin coating process and the controlled transfer to (and only to) the intended locations on the micromechanical structure. One beneft of this method is increased design flexibility due to extension of the range of combinations of conventional thin film technology with other materials such as polymer films. The method was tested by preparing small cavities and other simple three-dimensional microstructures where it was mandatory to prevent the adhesive filling up the voids between the contact areas. Examples of composite structures made of 0.6 ~m thick PET films glued to micropattemed structures on silicon substrates are presented. © 1998 Elsevier Science S.A. Keywords: Adhesion; Organic films; Polymer films; Silicon
1. I n t r o d u c t i o n In the production of micromechanical devices adhesives are often employed for mounting purposes only but not for the manufacture of the micromechanical structures themselves. One main reason is that assembling micromechanical parts with adhesives requires precise appIication of very small volumes of the glue to exactly defined areas of the sample surface. Commonly used methods for applying small volumes of adhesives are applied for surface mounting electronic parts and allow doses not much less than 1 nl. Novel methods using piezoceramic actuators coinparable with those in ink jet printers have been shown to handle volumes down to 5 pl [ 1]. Adhesive films for microsystem assembly may require even lower volumes and a thickness comparable with the respective dimensions in thin film technology which can easily be in the range of 100 nm. Here we report on micromechanicaI structures manufactured from thin polymer foils and silicon microstructures with a novel method for micromechanical adhesive joints. The requirements for these adhesive joints and the adhesive used are as follows. 0928-4931/98/$19.00 © 1998 Elsevier Science S.A. AII rights reserved PIIS0928-4931 ( 97)00048-9
• The joint strength must be satisfactory for the application. • In order to glue micromechanical structures produced using thin film processes, a thickness of the adhesive layer of less than a micrometer is to be achieved. • The method of applying the adhesive should allow control of the areas to be coated on a micrometer scale. Volumes to be applied onto a given contact area thus can be much less than a picoliter. The process employed to fabricate adhesive joints according to these specifications consists of two steps. l. A spin coating process is used to obtain a thin film of the adhesive on a substrate. The thickness of the layer can be controlled by adjusting the initial viscosity of the adhesive and the spin speed. 2. A stamp printing transfer of the adhesive layer onto the elevated features of the structure is performed. Here, the structure to be coated plays the role of the stamp itself. The stamp printing of the adhesive poses an additional restriction on the choice of glue: its theological behavior is required not to drag fathoms when the stamp is removed from the spin coated surface.
228
H. Dreuth, C. [-leiden / Ma;erials Science and Engineering C 5 (t998) 227-231
2. Experimental details
state and therefore no heat cure of the joint is required, they remain sticky at room temperature for days (the glass temperature of the polymer is about - 5 5 °C, see [3], p. 230). This adhesive also can be arbitrarily diluted in various solvents to obtain low viscosities as required for the spin process (the viscosity has to be in the order of 1 mPa s). The adhesion mechanism between this adhesive and the corresponding adherends involves only physical interactions ( dipol and dispersive forces). One drawback of this adhesive is its smaller adhesiveness compared with chemical reactive adhesives. Whether the adhesiveness is sufficient for the intended use is therefore to be determined. In our case where parts of thin polymer foil are involved, the strength of the adhesive joint was acceptable since the very thin polymer foil itself can onty withstand small absolute forces.
Owing to its favorable electrostatic properties and because of the availability of high quality thin films, polyethylene terephthalate (PET) was chosen as polymer. Thin membranes of PET (down to a thickness of 0.6 b~m) were to be bonded adhesively to microstructures on silicon wafers.
2.1, The selection of the adhesive In order to create a thin adhesive film on a flat substrate by a spin process, suitable viscosity of the adhesive is essential. Therefore the main criterion for the choice of adhesive was that the viscosity of this adhesive mass can easily be tuned by adjusting the fraction of diluting solvent. Second, heat curing of a reactive adhesive was not acceptable because of the glass temperature of PET which is only 65-68 °C [2] (see [ 3 ], p. 287). Third, the thin adhesive layer prepared by spinning had to remain sticky at least for the time necessary to complete the adhesive joint and should not for example harden prematurely owing to its small volume and large surface. Also diffusion adhesion (see [4], p. 428ff; [5], p. 341 ) is not an appropriate method for bonding PET because PET exhibits good chemical resistance (see [5], p. 414). The adhesive chosen was polybutyl acrylate diluted in aceton/benzine (manufactured by Beiersdorf AG, Hamburg) ; it is usually used for the production of adhesive tape. Polybutyl acrylate fulfils the above requirements. While its polymer molecules are already present in their final chemical
2.2. Process description The process illustrated schematically in Fig. 1 involves several steps. The microstructures onto which the PET foil was to be glued were manufactured as follows. First a photoresist mask was structured on a silicon substrate by standard lithographical methods, Second a layer of tungsten titanium (thickness about 600 nm) was sputtered onto the sample, Then a lift-off process in acetone completed the structures, To avoid uncontrolled deposition of glue between the microstmctures, the thickness of the adhesive layer should be less than the height of the microstucture, In order to achieve layers less than 1 p,m thick by spinning the adhesive onto a
substrate
h\ structure
adhesive
~
r,,
..~ubs,tra-tez
t/
I. Substrates brought into contact
PETfoil
adhesive
? II. Pressure applied TFI,Substrates separated
~ f r - - v ~-f/
IV:.PET foil joined to substrate V, Completed Structure Fig. 1. Schematicdiagramof the process.
H. Dreuth, C. Heiden/Materials Science and Engineering C 5 (1998) 227-231
substrate, it was necess~y to dilute the original adhesive mass whose solid fraction amounted to 44%. Tile solvents tested were acetone and ethyl acetate. With these solvents the original adhesive mass was diluted by about 1 part of the adhesive mass to 30 parts of the respective solvent. This solution then was spun onto the substrates at 1000-5000 rev min -l. The thin adhesive layer was then transfered onto the elevated microstructures by pressing the two corresponding substrates together. The pressure is not applied primarily to increase the effective area of every adhesive joint itself or to ensure a uniforin thickness of the adhesive joint, but to achieve close contact between the wafer with the thin adhesive layer and all the elevated structures of the sample. The amount of pressure has to be adjusted such that first it is high enough that all elevated structures are in contact with the adhesive layer, and second the substrates are not bent too much. Bending the substrate could otherwise cause the adhesive layer to touch parts below the elevated structures or adhesive to be squeezed out of the joint areas. To apply the appropriate pressure a vice was used. The jaws that press onto the substrates were attached with an additional thin layer of rubber to even out mechanical inaccuracies of the arrangement. The force exerted on the samples could be adjusted by applying the appropriate torque to the vice. The transfer of the adhesive layer to the elevated structures was then completed by separating the two substrates. To attach the thin polymer foil to the sample the foil was carried by a frame. The two pieces were then simply put together to form the adhesive bond. To prevent the foil from being stretched between the contours of the microstructure, the pressure was not applied mechanically onto the foil. Instead a stream of nitrogen was applied to achieve contact between the foil and the adhesive. Since the adhesive only resided on the elevated structures, the foil only adhered there, leaving cavities between these structures. Because of the dimensions of the structures and adhesive layers, all these processes were performed in a cleanroom environment.
2.3. Process parameters Various parameters can influence the quality of the process described above. Most important are the following: • the type of substrate for spin coating of the adhesive, • the spin speed (which determines the thickness of the adhesive layer), • the solvent used for diluting the adhesive, • the stamp printing pressure, • the mechanical arrangement for the stamp printing of the adhesive, • the geometry of the microstructures to be coated. For the tests simple geometries such as stripe patterns were used for the microstructures. They are characterized by their aspect ratio which is defined as the ratio of their height and the distance between them.
229
As substrates for the adhesive usually silicon wafers were used. For comparison also polished PTFE substrates were used to evaluate whether a material with very low surface energy would facilitate the transfer of the adhesive layer.
3. Results
3.1. Properties of the spun adhesive layers Polybutyl acrylate showed different properties when diluted either in acetone or in ethyl acetate. While a solution in acetone did not yield as homogeneous adhesive layers as a solution in ethyl acetate, the latter showed more tendency to draw threads when the substrates are separated. With this solution the thickness of the adhesive film could be varied in the range from about 200 nm down to less then 100 nm with satisfactory homogeneity. The thickness was estimated through the film interference colors. The substrates the adhesive was spun onto have to be as smooth as possible in order to allow a successful transfer of the adhesive layer to the microstructures. Mostly polished silicon wafers were used for this purpose. A test with polished polytetrafluoroethylene (PTFE) substrates showed that the adhesive layer could be transfered to the microstructures more completely than if it were transfered from a silicon substrate. This has been attributed to the very low surface energy of PTFE which is only 18.5 mJ m -2 (see [4], p. 235). Although acetone and ethyl acetate both exhibit higher surface energies of about 24 mJ m - 2 at room temperature (see [6], p. F-33), spin coating of the whole adhesive solution could be performed successfully. Nevertheless, the use of PTFE substrates was discontinued because the relative softness of PTFE compared with silicon made the stamp printing transfer of the adhesive onto the microstructures more sensitive to variations in parameters such as the pressure applied during the transfer, leading to a greater chance that adhesive would be spilled in between the structures. The time elapsed between the spin coating process and the stamp printing of the adhesive was varied between about 5 rain and 1 day. No significant effect of these time intervals on the quality of the process was observed. This corresponds to the fact that the polybutyl acrylate adhesive layer retains its stickyness for even longer periods of time.
3.2. Importa~zt parameters of the stamp printing process To perform a successful transfer of the thin adhesive layer from the substrate to the microstructures two important points had to be taken into account. The most critical parameter was the pressure to be applied. It had to have a suitable value and also needed to be applied uniformly over the whole sample. On the other hand it was also necessary to avoid lateral movements both when the substrates were first put into contact and when they were seperated. Otherwise additional spilling of the adhesive could occur. Appropriate pressures exerted onto
230
~
H. Dreuth, C. Heiden/Materials Science and Engineering C 5 (1998) 227-231
-
: ~. ~ ~
r-~.tl~ll~itt,;{
~i ] : k
~
i°~ ~, ~,
i. . . . . ,
}-
Fig. 2.3 lxm stripes correctlycoated with adhesive. the substrates were found to be on the order of some 1 X 10 6 Pa. It turned out that the mechanical arrangement using a vice for controlling the stamp printing was not sound enough to ensure constant quality of the process. The separating process was performed manually and was also a possible cause of lateral movements when the two substrates were in contact, which causes adhesive to be moved from the elevated structures to locations where it should not be deposited. Fig. 2 shows a sample where the 3 ~m stripes are correctly coated with the adhesive after the stamp printing process. In Fig. 3 another part of the same sample demonstrates that lateral movement during the contact of the substrates can cause parts of the adhesive to be drawn across the sample. 3.3. Influence o f the geometry o f the microstructures
Depending on the aspect ratio of the microstructure two different problems were encountered. First, at high aspect ratios (where the microstructures are located relatively close to each other) the results are especially sensitive even to small amounts of adhesive spilled from those structures between them. This might be tolerable in the second case where the distance between the structures is relatively large (low aspect ratio). Here the chances are that owing to too much pressure during the stamp printing process the two substrates touch
Fig. 4. Completedstructure with high aspect ratio. each other not only at the elevated microstructures but also on some areas in between. The optimal pressure thus also depends on the aspect ratio. The samples used here had aspect ratios between 0.2 (3 Ixm distance between the stripes with thickness of about 0.6 ~Lm) and 0.00057 ( 1 mm distance between structures of 0.57 p~m height). Fig. 4 shows a SEM picture of a sample where the tungsten titanium spacers have a width of 10 p~m. While a 0.9 ~Lm thick PET foil is attached to these spacers the volume between them remains void. This sample was prepared using the techniques described before. Then the silicon wafer was broken and the PET foil torn off at the edge to allow inspection of the cavities. The spatial uniformity of the adhesive transfer on the area of the whole sample depends strongly on the mechanical precision with which the transfer was carried out and also on the other various parameters described above. Sufficient uniformity was achieved on samples where both substrates consisted of whole wafers (of 2 inch diaineter) with low aspect ratio structures. Mechanical inaccuracies of the stamp printing then did not influence the results to the extent that they did on small samples where the uniformity was not satisfactory. Improvements in the mechanical arrangement of the stamp print process are expected to solve this problem.
4. Conclusions
Fig. 3, Adhesive drawn across the 3 gm stripes•
The method described in this paper allows the micromechanical combination of materials which are usually processed with very different methods that cannot be applied equally to both types of material. The use of thin PET foils allowed very thin membranes of relative large area to be attached onto micromechanical structures which would be very difficult if only thin film technology were used. To bond the foil onto the micropatterned structures without filling the area between them, the volumes of the adhesive to be applied had to be lower than could be applied using direct dispensing
H. Dreuth, C. Heiden / Materials Science and Engineering C 5 (1998) 227-231
techniques or combinations of dispensing and then stamp printing the adhesive. A combination of a spin coating process of an adhesive layer together with a stamp printing technique permitted reduction of the volumes applied on a single adhesive joint from the nanoliter range down to well below picoliters. Although this method can be applied to various combinations of materials and possible applications, it must be realized that there are requirements to be met that not every adherend/ adhesive system fulfils. Restrictions here include chemical and physical properties of the adhesive and geometrical properties of the microstructures as well. To enhance the reliability of the stamp printing process, optimization of the mechanical arrangement will be required. Here coating of adhesives was only performed on elevated structures, using these structures directly as a stamp. Applying adhesives onto areas not elevated compared with the surrounding structures in a controlled way would require an
231
additional step where an auxilliary stamp transfers the adhesive onto the designated areas. This stamp must be designed according to the geometry used. Work towards joining adhesively micromechanical structures made of thin polymer foils only is in progress.
References [1] A. Hardwig, M. D6ring, O.-D. Hennemann, Kleben und Dichten, Jahrgang 40, 7-8, 1996, pp. 31-33. [2] R.N. Haward, Colloid Polym. Sci., 258 (1980) 643-662. [3] J.M.G. Cowie. Chemie und Physik der Polymeren, Verlag Chemie OmbH, Weinheiin, 1976. [4] G. Habenicht, Kleben: Grundlagen, Technologie, Anwendungen, Springer, Berlin, 1990. [ 5 ] A.V. Pocius, R.D. Waid, S.R. Hartshorn, in J.D. Minford (ed.), Treatise on Adhesion and Adhesives, VoL 7, Marcel Dekker, New York, 1991, pp. 333-435. [ 6 ] R.C. Weast, M.J. Astle, W.H. Beyer (Eds.), Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1987, 67th edn.