Mask materials for powder blasting

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J. Micromech. Microeng. 10 (2000) 175 180.

Box 217, 7500 AE Enschede, The Netherlands E-mail: h.wensink@tn.utwente.nl Received 13 December 1999 Abstract.

1 J. Micromech. Microeng. 10 (2000) Printed in the UK PII: S (00)09330-X Mask materials for powder blasting Henk Wensink, Henri V Jansen, J W Berenschot and Miko C Elwenspoek MESA + Research Institute, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands h.wensink@tn.utwente.nl Received 13 December 1999 Abstract. Powder blasting, or abrasive jet machining (AJM), is a technique in which a particle jet is directed towards a target for mechanical material removal. It is a fast, cheap and accurate directional etch technique for brittle materials such as glass, silicon and ceramics. The particle jet (which expands to about 1 cm in diameter) can be optimized for etching, while the mask defines the small and complex structures. The quality of the mask influences the performance of powder blasting. In this study we tested and compared several mask types and added a new one: electroplated copper. The latter combines a highly resistant mask material for powder blasting with the high-resolution capabilities of lithography, which makes it possible to obtain an accurate pattern transfer and small feature sizes (<50 µm). M This article features multimedia enhancements available from the abstract page in the online journal; see 1. Introduction Sand blasting is an old technique that is used to remove paint, clean houses and decorate glass. Powder blasting, with small (<100 µm) particles, is used, for example, for device demarking in the electronics industry, surface preparation prior to plating (without mask) but also rapid prototyping [1] and flat panel display production [2, 3] (with mask). In research, it is widely used to determine the wear rate of industrial materials. Powder blasting can also be used for micromachining. Being an IC technology spin off, micromachining is normally performed on silicon whereas other materials are less frequently processed. Powder blasting is suitable for machining a wide range of materials, such as glass, silicon and ceramics, and the required equipment is relatively cheap. It is a fast process; the time to etch through a 500 µm thick 3 Pyrex wafer with one nozzle is approximately 20 min in our set-up. It also fits very well between the common micromachining techniques due to its lithographic masking abilities, process similarities and compatibility. For the production of flat panel displays, powder blasting was judged to be the best process to create thousands of holes at once at low cost, high speed and with a high accuracy [2]. In the field of micromechanics, powder blasting has already successfully been used, for example, for making an inertial sensor [4], a self-priming peristaltic micropump [5] and a miniaturized capillary electrophoresis chip [6]. There are several advantages to using a mask with powder blasting. The particle jet (which expands to about 1 cm in diameter) can be optimized for etching, while the mask defines the small and complex structures. It also allows multiple jets to be used, which decreases the process time. Figure 1. A schematic diagram of the powder blast process. Figure 2. Schematic diagram of the powder blasting set-up /00/ $ IOP Publishing Ltd 175

First, it requires the use of an average particle size of at most a third of the desired channel width (as a rule of thumb).

2 H Wensink et al Figure 3. Perspective view of a typical etch result with powder blasting in Pyrex glass (400 µm deep). Up to now, the minimum feature size with powder blasting is limited in practice to about 100 µm [7], a major goal of our project is to decrease it. For this, two things are important. First, it requires the use of an average particle size of at most a third of the desired channel width (as a rule of thumb). Fortunately, sharp alumina (Al 2 O 3 ) particles are available in large quantities with any average size. Second, it has to be possible to make a mask with small feature sizes. In this article we look at mask materials for powder blasting in general, while testing them with 30 µm alumina particles, and for the possibility of making smaller dimensions in particular. 2. Powder blasting Powder blasting is a technology in which a particle jet is directed towards a target for mechanical material removal (figure 1). The particles are accelerated towards the target with a high-pressure airflow (figure 2). The airflow is mixed with the particles by a vibrating feeder (HP-2, Texas Airsonics). The mixture is directed through a circular nozzle (with a diameter of 1.5 mm) at the end of the tube. The particles hit the target with a speed of m s 1 (depending on the air pressure) in a separate box. This box is ventilated by a cyclone, which removes the particles from the airflow. A lateral movement ensures an evenly etched surface while a mask, which contains the design, covers the target. Figure 3 shows a typical result of powder blasting in Pyrex glass. The etch process is usually described as the sum of many single-particle impacts. There are roughly three classes of materials to be considered in powder blasting; ductile materials (metals), elastomers (rubbers) and brittle materials (glass silicon and ceramics). Ductile materials erode by cutting and ploughing, elastomers by fatigue mechanisms and brittle materials by crack growth. When a brittle material is impacted by a hard sharp particle, the contact area is Figure 4. Etch mechanism of a single-particle impact. M An animated GIF of this figure is available from the article s abstract page in the online journal; see plastically deformed due to the resulting high compressive stress. This deformation leads to large tensile stresses after the impact (relaxation) that result in lateral cracks causing the material removal (figure 4) and a roughness of µm R a (depending on the kinetic energy of the particles [8]). The etch mechanism of a single particle impact is more thoroughly described in erosion related papers [8, 9]. Figure 5 shows that ductile materials (and also elastomers [10]) have a low erosion rate, compared to brittle materials, with a local minimum at 90 impact angle [11]. Since the particle jet is directed perpendicular towards the target during blasting for maximum brittle erosion, ductile materials and elastomers can be suitable for use as masks. 3. Mask materials The main qualification for a good mask material is a low erosion rate. We also look for the capability of an accurate and easy pattern transfer, and the possibility to create small feature sizes. We first test two polymers that can be 176

3 Mask materials for powder blasting Figure 6. Erosion of BF405 foil. Figure 5. Two basic etch characteristics in powder blasting (90 is perpendicular impact). After [10]. lithographically structured and, second, we examined two metal masks Powder blasting foil Ordyl BF400 is an elastic negative resist foil fabricated by Tokyo Ohka Kogyo (Japan), and specially developed for powder blasting [3, 11]. There are two types: 50 µm (BF405) and 100 µm (BF410) thick, we tested the first one. The foil is applied to the target on a hot-plate (105 C). After exposure (150 mj cm 2 ), it is spray developed with 0.2% Na 2 CO 3. The resulting layer was found to be 43 µm thick. After blasting, the foil can be removed with a 10% KOH solution at room temperature, or, for example, with a HNO 3 solution. In figure 6 the decrease in thickness of BF405 is shown when eroded by alumina particles with an average size of 30 µm and a speed of 180 m s 1. The thickness was measured with a Dektak profiler (Sloan DEKTAK II), after blowing away the remaining particles with dry nitrogen. In the beginning of the erosion, the thickness of BF405 does not seem to decrease at all (a small increase is even observed). The foil is first degraded by the particle impacts creating subsurface cracks without actually being removed. After this incubation time the damage becomes so severe that further blasting does decrease the resist thickness. Steady-state erosion is reached, and the thickness decreases linearly. This effect is well known and has also been observed by others [12]. The erosion was monitored until it had a thickness of about 17 µm. Now the particles were able to penetrate the resist and damage the target. From this point on, both the target and resist are rapidly removed. This restricts the maximum etch depth with this foil and these conditions to about 350 µm in Pyrex glass. Already, before this breakdown, some particles are able to penetrate the foil deep enough to hit the target at undesired spots. Figure 7 shows such damage. Therefore, the foil does not completely protect the target from the particles. When etching channels, another important effect occurs. The resist edges that define the channel are more vulnerable to the particle impacts and are readily eroded. Therefore, the top width of a channel increases during blasting, as shown in figure 8. When applying a postbake of 150 C, these effects are reduced to some extent Polyimide Polyimide is a negative liquid resist that is very well known for its high strength, we tested Durimide 7510 (Arch Chemicals). A thick layer was applied by spinning at 660 rpm. After exposure (300 mj cm 2 ), development and a 1 h anneal at 350 C, the thickness was 35 µm. The erosion rate was tested by blasting the sample with alumina particles with an average size of 30 µm at a speed of 180 m s 1. The decrease in thickness was measured using a Dektak surface profiler. The polyimide clearly showed a higher steady-state erosion compared to BF405, approximately by a factor of two. It also had a shorter incubation time before the steadystate erosion was reached, which indicates a more brittle behaviour [13]. This high erosion rate makes polyimide less suitable for a mask material in deep and accurate powder blasting Metal plate masks Metals are normally used as a powder blasting mask material by means of a metal plate (e.g. stainless-steel) [2, 4]. By drilling, milling or laser machining a pattern is created in this mask. The mask can simply be clamped directly together with the target. However, since a plate is never completely flat, there will be voids underneath the mask (figure 9). This will allow the particles to get into the voids and damage the target. Also, the impact of particles on the top surface of the plate will induce great stresses, which can result in buckling of the mask (especially with very thin plates). To prevent this, the mask can be clamped magnetically to the target or an intermediate protection/adhesion layer can be applied [2]. When, in the latter case, this extra layer also covers the blast area, it should easily be removed by powder blasting. 177

H Wensink et al Figure 7. Photograph showing the damage occurring underneath the BF405 foil. Figure 8. The channel width increases during blasting. Figure 9.

Wax seal (W100, Agar Scientific Ltd, Stansted, UK) meets the requirements to be an intermediate protection/adhesion layer. It is a brittle material, which becomes viscous at higher temperatures.

4 H Wensink et al Figure 7. Photograph showing the damage occurring underneath the BF405 foil. Figure 8. The channel width increases during blasting. Figure 9. Schematic diagram showing how a metal plate mask clamped to the target introduces voids. Wax seal (W100, Agar Scientific Ltd, Stansted, UK) meets the requirements to be an intermediate protection/adhesion layer. It is a brittle material, which becomes viscous at higher temperatures. At a temperature of 150 C the wax can be spread on the (also heated) mask. The target is placed on the mask and the stack is cooled to room temperature. This results in a very tight and strong connection. Since the wax is brittle, it is very easily removed by sandblasting and it has practically no influence on uniformity (figure 10). After blasting, reheating separates the mask from the target and both can be cleaned with chloroform. The positive effect of an intermediate layer is clearly seen in figure 11. A relatively thick metal layer lasts for a long time, so a mask can be used on several targets Copper In order to combine the low erosion rate of a metal, and the high resolution of a lithographic process, we decided to apply a metal mask on the target by electroplating. Copper was chosen as the metal because of its wide use in electroplating and its high wear resistance against powder blasting [9]. Good adhesion with the target is important, hence an intermediate titanium adhesion layer is used between the target and the copper. Additionally, the target is cleaned with an oxygen plasma before sputtering the metal layers. It was observed that without these precautions, the copper mask buckled and was released from the Pyrex glass target during powder blasting. (a) Figure 10. Schematic diagram of a metal mask on a target with adhesion/protection layer (a) and how the layer prevents ingress of particles (b). Figure 12 shows how copper is applied as a mask. First, the whole target is covered by a titanium (15 nm)/copper (400 nm) seed layer by sputtering, step (1). Second, a thick polymer foil that is commonly used with electroplating (Ordyl Alpha440T, Tokyo Ohka Kogyo) is applied, step (2), and lithographically defined, step (3). Now the target is plated with copper, step (4), after which the polymer is removed with a 10% KOH solution at room temperature, step (5). The thin seed layer beneath the resist mould is generally not removed separately, but is easily etched away during the blasting, step (6). After blasting, the remaining copper can be removed with a strong acid, such as HNO 3. When blasting with particles with an average size of 30 µm at a speed of 180 m s 1, the erosion rate of the copper was found to be about seven times lower compared to the (b) 178

Mask materials for powder blasting Table 1. Overview of the mask types used (tested with 30 µm alumina particle at a speed of 180 m s 1 ).

11. Etching a 1 mm hole using a 1 mm metal mask: the resulting top views are when etched with (a) and without (b) an adhesion/protection layer. (b) Figure 13.

The masks were removed for both pictures. a channel are not very vulnerable to particle impacts. The channel width decreases only a few micrometres after erosion with more than 3.5gcm 2.

Although these profiles are, in this case, blasted with different particle sizes, it gives a valid general idea of the in-accurate pattern transfer of a BF405 mask. 4. Discussion Figure 12.

Also, even with a thin copper layer (20 µm), no particles were able to penetrate the mask and damage the target.

5 Mask materials for powder blasting Table 1. Overview of the mask types used (tested with 30 µm alumina particle at a speed of 180 m s 1 ). Mask type Minimum dimension Erosion rate Remarks Polyimide <10 µm High BF 405 >50 µm Intermediate Metal plate >50 µm Low Pattern constraints Electroplated copper <10 µm Low (a) (a) (b) Figure 11. Etching a 1 mm hole using a 1 mm metal mask: the resulting top views are when etched with (a) and without (b) an adhesion/protection layer. (b) Figure 13. The influence of the mask on the channel profile: with a BF405 mask and 30 µm particles (a) and with a 50 µm thick electroplated copper mask and 9 µm particles (b). The masks were removed for both pictures. a channel are not very vulnerable to particle impacts. The channel width decreases only a few micrometres after erosion with more than 3.5gcm 2. The difference in channel profile when powder blasting with BF405 or electroplated copper is clearly seen in figure 13. Although these profiles are, in this case, blasted with different particle sizes, it gives a valid general idea of the in-accurate pattern transfer of a BF405 mask. 4. Discussion Figure 12. Schematic showing the steps in applying a copper mask. BF405 steady-state erosion (i.e. a selectivity with Pyrex glass of about 65). Also, even with a thin copper layer (20 µm), no particles were able to penetrate the mask and damage the target. By using a 50 µm thick mask, the edges that define The powder blasting foil Ordyl BF405 (50 µm thick) has a reasonable low erosion rate, is easy to use and very suitable for standard blasting applications with complex designs. With lithography, only trenches of 70 µm wide or more can be achieved with the 50 µm thick BF405 due to the thickness and properties of the resist. When using polyimide, the mask thickness can be adjusted so a very small feature size can be obtained. However, although polyimide is known for its strength, the 179

6 H Wensink et al erosion rate is higher which makes it not very suitable for a mask material for deep and accurate powder blasting. A metal plate mask has the advantages of a low erosion rate and its applicability on any kind of material. However, the disadvantages are the limitations in feature size (approximately 50 µm when laser machining in a 0.5 mm thick plate [4]) and pattern constraints (ring patterns cannot be used because the inside needs to be supported). By creating a mask with electroplating, we can combine the low erosion rate of metals with the high resolution of a lithographic process. This also allows adjustment, the desired thickness of the mask, which can depend on process requirements. For plating, copper was used as a mask material because of its low erosion rate and wide use in electroplating. More importantly, copper edges are also very erosion resistant, which results in very accurate pattern transfer. With an electroplated copper mask, the influence of the mask on the shape of the powder blasted structures (e.g. channel definition and profile) is minimized. 5. Conclusions Powder blasting is a fast, cheap and accurate directional etch technique for brittle materials such as glass, silicon and ceramics. There are many possibilities with pattern transfer in powder blasting, which gives this micro technology the perspective to become a new standard tool in micromachining. The accuracy of the process also depends on the type of mask that is used. In this paper, several mask types were tested for this application. An overview of their properties is listed in table 1. The negative resist foil BF405 has a reasonable low erosion rate, is easy to use and very suitable for standard blasting applications with complex designs. The minimum feature size of this foil using lithography is restricted by its properties and standard thickness of 50 µm to about 70 µm. With polyimide, the minimum feature size can be smaller, but the erosion rate of this negative resist is higher. A metal plate mask has a low erosion rate, but is limited in feature size and it has pattern constraints. Electroplated copper was introduced as a new mask material for powder blasting which combines the low erosion rate of a metal with the high-resolution capabilities of a lithographic process. It is very suitable for deep and accurate powder blasting, and has the opportunity to be a suitable mask for machining feature sizes of less than 50 µm. Acknowledgments This work is part of the research programme of the Stichting voor Fundamenteel Onderzoek der Materie (FOM), which is financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). References [1] Kruusing A, Leppävuori S, Uusimäki A and Uusimäki M 1999 Rapid prototyping of silicon structures by aid of laser and abrasive-jet machining Proc. SPIE [2] Ligthart H J, Slikkerveer P J, in t Veld F H, Swinkels P H W and Zonneveld M H 1996 Glass and glass machining in ZEUS panels Philips J. Res [3] Fujii H, Tanabe H, Ishiga H, Harayama M and Oka M 1992 A sandblasting process for fabrication of color PDP phosphor screens SID 92 Digest [4] Belloy E, Thurre S, Walckiers E, Sayah A and Gijs MAM 1999 Powder blasting as a new technology for inertial sensor fabrication Eurosensors XIII (The Hague, The Netherlands, 1999) pp [5] Veenstra T T, Berenschot J W, Sanders RGP, Gardeniers JGE,Elwenspoek M C and van den Berg A 2000 A simple selfpriming bubble-tolerant peristaltic micropump Eurosensors XIV (Copenhagen, Denmark, 2000) [6] Schasfoort R, Guijt-van Duijn R, Schlautmann S, Frank H, Billiet H, van Dedem G and van den Berg A 2000 Miniaturized capillary electrophoresis system with integrated conductivity detector µtas 2000 (Enschede, The Netherlands, 2000) [7] Slikkerveer P J, BoutenPCPanddeHaasFCM1999 High quality mechanical etching of brittle materials by powder blasting (Eurosensors XIII, The Netherlands) Sensors Actuators pp [8] Slikkerveer P J, Bouten PCP,in tveldfhandscholten H 1998 Erosion and damage by sharp particles Wear [9] Finnie I 1995 Some reflections on the past and future of erosion Wear 186/ [10] Slikkerveer P J, van Dongen MHAandTouwslager F J 1999 Erosion of elastomeric protective coatings Wear [11] Ritter J E (ed) 1992 Erosion of Ceramic Materials (Zurich: Trans Tech) [12] Arnold J C and Hutchings I M 1990 The mechanisms of erosion of unfilled elastomers by solid particle impact Wear [13] Friedrich K 1986 Erosive wear of polymer surfaces by steel ball blasting J. Mater. Sci

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