Precision manufacturing methods of inserts for injection molding of microfluidic systems.

Giuliano Bissacco, Hans N. Hansen, Peter T. Tang & Jimmy Fugl

Department of Manufacturing Engineering and Management Technical University of Denmark

Produktionstorvet, Kgs. 2800 Lyngby, Denmark

Abstract Manufacturing of tools for micro injection molding and hot embossing of microfluidic systems can be realized by several different process sequences, mainly based on combinations of photolithography, etching and electrodeposition. The paper presents a classification of the available process sequence alternatives and proposes a new solution based on generation of the pattern by mechanical or thermal material removal operations. An experimental investigation is carried out for the verification of the accuracy achievable with the proposed method. Experimental results are compared to literature data concerning the other fabrication alternatives. Although mechanical and thermal material removal operations are inferior to conventional methods regarding accuracy and minimum feature size, they allow the integration of real 3D features into the design of polymer microfluidic systems to be produced in large volumes. Introduction Manufacturing of polymer microfluidic components is a key technology for the implementation of microfluidic devices in consumer products. If fluidic chips can be produced in an effective, accurate and economical way, a whole range of new applications will emerge and the applications already existing will attract much more attention. Low cost mass-production of polymer microfluidic components can be achieved using either injection molding or hot embossing. Both techniques require a tool to transfer the microstructures to the polymer material. A key issue is therefore the manufacturing of the tool. Depending on the number of replications specified and required accuracy, a number of very different processes can be utilized to obtain the tool (or tool insert in case of injection molding). Currently, production methods are mainly based on combinations of photolithography, etching and electrodeposition, and are therefore characterized by the limitations of such processes. Particularly the obtainable geometries are limited to 2½D microstructures. A viable alternative is constituted by the introduction of mechanical and thermal processes based on material removal in the process chain for insert fabrication. Such processes allow manufacturing of complex 3D shapes with high aspect ratios. Basic tool fabrication schemes The currently known methods for fabrication of tools for hot embossing and injection molding can be grouped in four basic manufacturing schemes. Some of the schemes include variants so the total number of fabrication schemes amounts to eight. Of the four main fabrication schemes two use silicon substrates and two use other materials as the substrate (polymers or metals). Two schemes utilize subtractive micromachining (reactive ion etching, mechanical micromachining or laser micromachining) and two schemes involve additive micromachining (UV-lithography combined with electroforming). The four schemes are briefly presented in the following.

Fabrication scheme 1 A thick photoresist is applied to a flat metal disc by spin coating. The photoresist (such as SU-8) is exposed through a mask and developed. The metal substrate is then chemically or electrochemically activated to remove residues of photoresist and oxide layers. The disc is then moved to the electroplating bath and a nickel layer is built having a thickness just below that of the photoresist. The

photoresist is stripped and the tool insert is complete. The process sequence is described in [1] and shown in Fig. 1a. This process looks a little like UV-LIGA, except that the tool insert is fabricated “directly” and thus becomes a “negative” as compared to UV-LIGA.

Fabrication scheme 2 This scheme is similar to what is sometimes referred to as UV-LIGA because the hard X-ray radiation used in traditional LIGA [2] is replaced with cheap and relatively long wavelength UV-light. The substrate is a clean, oxidized silicon wafer with a plating base consisting of 50/500 Å of Ti/Au (or Cr/Au) deposited by sputtering or evaporation of the metals in a vacuum chamber (PVD). A thick photoresist (such as SU-8) is applied to the disc by spin coating, exposed through a mask and developed. Photoresist residues are then removed by exposing the entire wafer to low energy oxygen plasma. At this point the fabrication scheme can move in two directions, Fig. 1b: Scheme 2a: The disc is moved to the electroplating bath and a nickel layer is built having a thickness just below that of the photoresist. In order to make the photoresist conducting on the top, the disc is returned to the vacuum chamber and a layer of for instance chromium/gold is sputtered onto the entire surface. The electroplating is then continued until a thickness of 2-3 mm is reached. Finally the nickel insert machined to the desired size and flatness, the silicon wafer dissolved in KOH (potassium hydroxide) and the photoresist removed. Scheme 2b: The wafer is electroplated with several millimeters of nickel - the nickel structures are simply allowed to grow together eventually covering the entire wafer. This is followed by machining of the back of the wafer and selective etching of silicon, plating base and photoresist. The basic difference between schemes 2a and 2b consists in the introduction of a conductive layer in order to uniform the growth of the nickel on the back of the insert.

Fabrication scheme 3 Differently from the previous schemes, this one is based on subtractive micromachining by means of Reactive Ion Etching (RIE), also called Advanced Silicon Etching (AES). The pattern is obtained by UV-lithography of a thin photoresist (such as Hoechst AZ5214E) and subsequent etching of a mask material (typically silicon oxide or silicon nitride). Depending on whether, after the etching, the silicon wafer is to be used directly as a tool, or indirectly as a master, a positive or a negative mask is used respectively, leading to the following variants, Fig. 1c: Scheme 3a: A thin layer of titanium and gold is sputtered onto the entire surface (including the side-walls). The wafer is then moved to the electroplating bath, and a metal layer of 2-3 mm is deposited. The tool insert is machined to the desired size and the silicon wafer is dissolved in KOH. The metal tool insert can be used for both hot-embossing and injection molding. The fabrication method is sometimes called DEEMO after Elders, Jansen and Elwenspoek [3]. Scheme 3b: The silicon wafer is glued or bonded onto a thick substrate with a similar thermal expansion coefficient (such as glass). This tool is used directly for hot-embossing.

Fabrication scheme 4 The required pattern is generated into a substrate by means of mechanical or thermal processes as for instance micromilling, microEDM or laser ablation. Depending on the chosen substrate and machining process, once the pattern has been machined, the fabrication scheme can move in one of three directions, Fig. 1d: Scheme 4a: The machined substrate, made of a polymer (such as ABS) or a soft-metal, is used as a master (positive) for electroforming of a tool insert (negative). If the substrate is made of polymer, chemical activation or coating by sputtering or evaporation is necessary prior to electroforming. The electroformed tool insert will typically consist of a thin activation layer (in case of a non-conducting polymer substrate), and a thick and wear resistant layer (nickel or nickel alloys). After electroforming and machining of the back of the insert, the substrate or master is dissolved. Scheme 4b: Here the substrate is machined and used “as it is”, perhaps with a little post-processing such as cleaning or mechanical polishing. If the substrate is a high performance polymer (e.g. PEEK), or a soft metal, the mould can be used for hot embossing [4]. If instead hardened tool steel is used for the substrate, the insert can be used for injection molding for very large volume production.

Scheme 4c: The substrate, made of polymer or soft-metal, is machined as in 4b and cleaned. To improve the wear and corrosion resistance it is then activated and electroplated with thin layers of copper, nickel or hard chromium. This scheme requires that metallization of the polymer or soft-metal is possible with good adhesion.

Fig. 1 Basic schemes for fabrication of hot embossing molds and injection molding mold inserts.

a) Fabrication sequence for a tool insert based on a metal substrate, lithography and electroforming.

b) Fabrication scheme based on silicon substrates, lithography and electroforming.

c) Fabrication schemes utilizing reactive ion etching of silicon, either indirectly (3A) or directly (3B) for the fabrication of tool inserts for hot-embossing and injection molding.

d) Fabrications schemes based on subtractive micromachining of non-silicon substrates.

While the first three fabrication schemes are basically limited as concerns the obtainable shapes to 2½D structures, the fourth one allows the generation of real 3D features on the inserts. Moreover, the manufacturing time is typically much shorter, as no photoresist needs to be applied, exposed, developed and removed and only one electroforming step is included (or none in case of direct machining on tool steel). On the other hand scheme 4 presents limitations concerning the minimum feature size and the overall achievable accuracy because of the inherent limitations of mechanical and thermal processes relative to UV-lithography. In the following, the results from an experimental investigation, based on the application of scheme 4a and 4b to the fabrication of injection molding mold inserts with 2½D and real 3D micro features, are presented. Experimental investigation on tool fabrication by mechanical material removal According to the fabrication scheme 4, the mold insert can be obtained by mechanical or thermal material removal operations, either directly on the substrate or on a master for subsequent electroforming. These two alternatives have been considered for an experimental investigation of the achievable accuracy of injection molding inserts. The selected process for material removal was micromilling. Micromilling is a suitable technique for manufacturing of microstructures characterized by high aspect ratios and complex geometries, allowing the realization of nearly any shape. The size of the features of the insert must comply with the minimum tool diameter. As a consequence, at present the minimum size of concave features is limited to approximately 100 microns. Convex features do not present the same limitation, but must be able to withstand to the cutting forces without excessive elastic deformation. Due to the low structural stiffness of the micro end mills, cutting forces produce not negligible deflections of the tool and thereby variations of the dimensions of the machined features relative to the nominal values. Finally, differently from etching processes where the material removal takes place at atomic level, in micromilling the minimum material removal unit is in the order of a few tens of µm3. This implies limitations on the resolution as well as problems related to material deformation, as for instance formation of burrs on the machined microstructures. The microfluidic system chosen for this investigation was not designed for a specific application, but as a test for the process capabilities. The insert design, which contains 2½D and 3D features with a minimum size of 200 microns, was chosen in such a way as to take advantage of the flexibility of the micromilling process, making it virtually impossible to produce it by silicon micromachining. The design incorporates four main chambers provided with micro features for fluid mixing and a series of channels connecting the chambers with each other and with the areas where the fluids are injected. The design of the plastic part is symmetric, as can be noticed from Fig. 2. The four chambers are 5 mm long, 1.5 mm wide and maximum depth is 1.5 mm. Two different types of protrusions characterize the four chambers. A series of three prismatic protrusions is present on the two right chambers in Fig. 2. Such protrusions are characterized by different heights, 1.2 mm, 0.9 mm and 0.6 mm. Similarly, three pyramidal protrusions (3D features) are present in each of the two left chambers, having heights of 1.2 mm, 0.9 mm and 0.6 mm. The channels are 0.2 mm both in width and depth. 12 complementary

chambers for fluid injection are present. Following scheme 4a, a master was realized in aluminum, its shape corresponding to the shape of the positive of the plastic part, while, according to scheme 4b, a mould insert, whose shape corresponds to the negative of the plastic part, was machined in pre-hardened tool steel (35 HRC). The machine tool used for the manufacturing of both the aluminum master and the tool steel insert was a 3 axis CNC vertical milling

3D pyramidal

2½D prismatic

Fig. 2 3D model of the test microfluidic system.

machine and high accuracy machining was achieved by means of a special machining procedure as described in [5]. The machining sequences required tools with diameters ranging from 6 mm to 200 µm. A detailed description of the machining sequences is reported in [6]. As regards the accuracy of the machined parts, dimensional verification was carried out on selected features [7]. The measured parameters were the width of the channels, measured with an optical CMM and the slope of the pyramids and height of prismatic protrusions in the main chambers, measured with a confocal microscope. The results are summarized in Tab. 1.

Tab. 1 Measurement results on selected features.

Al Steel Nominal Measured Deviation Measured Deviation channel 1 [µm] 200 191.7 -8.3 211.5 11.5 channel 2 [µm] 200 199.4 0.6 223.2 23.2 inter-channel [µm] 200 203.9 3.9 190.6 -9.4 pyramid slope [deg] 66.8 63.4 3.4 66.5 0.3 prism. protrus. height [µm] 900 906 6 899 -1

As for the channel area, Fig. 3, all the machined grooves, which correspond to the two channels for the metal masters and to the distance between the channels for the steel insert, are smaller than the nominal dimensions. This is due to the deviation of the tool diameter from the nominal value and to the development of tool wear. The protruding features instead, which correspond to the two channels for the steel insert and to the distance between the channels for the metal master, are larger than the nominal dimensions in both cases. For the metal master, this is the direct consequence of the reduced size of the two grooves. For the steel insert an additional effect due to the tool deflection during the side milling of the two channels, is present, which increases the observed deviations. Concerning the slope of the pyramids, deviations of 0.3° from the nominal slope were observed on the steel insert, while larger deviations of 3.4° were observed on the aluminum master. SEM images of the pyramids are shown in Fig. 4. Particularly accurate is the height of the prismatic protrusion on the steel insert, with deviations of 1 to 2 microns, Fig. 5. As a general observation on the accuracy of the micromilled features, grooves with nominal dimension equal to the nominal tool diameter show an overall good dimensional accuracy, while for the

vertical walls whose distance from the next feature is larger than the tool diameter, the accuracy in the plane orthogonal to the spindle axis is governed primarily by tool deflections and larger deviations are

Aluminum Steel

Fig. 3 SEM images of the channel area.

Aluminum Steel

Fig. 4 3D pyramidal protrusion/cavity area.

Aluminum Steel

Fig. 5 Prismatic protrusion/cavity area.

measured. For these features the deflection of the tool is allowed as there is no material to prevent it. In this respect, the tool steel insert is affected by higher tool deflections because of the higher hardness of the work material and because of a higher tool wear, which increase the cutting forces. Besides the dimensional and geometrical accuracy of the produced parts, a main issue was the presence of burrs. Higher burrs were observed on the aluminum master, while on the steel insert burrs were very high in some critical points, but lower than in the metal master elsewhere. A visual comparison of the burr type and height is given by the SEM images reported in figures from 3 to 5. Reduction of the burrs on the aluminum master was obtained by chemical etching with sodium hydroxide, while for the steel insert, manual mechanical removal was necessary. The remaining burrs on the aluminum master were replicated on the insert during electroforming (negative shape) and

reproduced on the molded plastic part. This is visible in Fig. 6 showing the channel area of a molded plastic part, with the replicated burrs on the right side of the channel. The presence of these undesired features on the replicated microfluidic systems constitutes a limitation of the capabilities of the insert manufacturing method, which are attributed solely to the chosen material removal technology. However, if a more onvenient method, as for instance

microEDM, is used no burrs would be produced and the replicated part would in principle be flawless.

c

Left side of the channel Right side of the channel

Fig. 6 Borders of a channel on the replicated plastic partobtained using the electroformed insert from the aluminum master. On the right side of the channel thereplicated burrs are clearly visible.

Comparison of manufacturing accuracy provided by the different schemes A comparison of the capabilities of the four fabrication schemes is shown in Tab. 2. The table is not complete. Despite the high accuracy provided by the first three schemes, regarding both position and dimensions of the features, which is in the order of 1 µm, they show a strong limitation regarding the type of shapes obtainable, with only 2½D features, perhaps stacked in not more than three layers. As

*For micromilling this value strongly depends on the type of material and feature (concave or convex).

Substrate material Metal Silicon Metal or polymer Method Photoresist ASE milling or laser Fabrication scheme 1 2a 2b 3a 3b 4a 4b 4c Geometry 2½D 2½D 2½D 2½D 2½D 3D 3D 3D Number of layers that can be stacked 1-3 1-3 1 1-3 1-3 ∞ ∞ ∞

XY (µm) 2 2 2 5 5 1-10* 1-20* 1-10* Features accuracy & alignment Z (µm) 1-5 1-5 1-5 1-5 1-5 3-10 3-10 3-10 Min. channel width (µm) 5 5 5 10 10 20-200 20-200 20-200 Max. channel depth (µm) 200 200 200 500 500 ∞** ∞** ∞** Max. aspect ratio 20 20 20 10 10 7.5 7.5 7.5

Range (°) 0-20 0-20 0-90 0-90 0-90 Side angle accuracy (°) 1-5 1-5 1-5 1-2 1-2 0.3 0.3 0.3 Surface roughness Sq Rms (nm) 200-300

** For micromilling this value depends on the tool diameter. Tab. 2 Comparison of the capabilities of the 4 basic fabrication schemes. Values refer to the average

capabilities of the single schemes.

demonstrated instead by the investigation above, pyramids, which can be regarded as an infinite number of 2D stacked patterns, can be easily generated by micromilling. Therefore, following the fabrication scheme 4, any 3D feature can be integrated in microfluidic systems designs. On the other hand, as shown in Tab. 2, this fabrication scheme is inferior to the others as concerns the dimensional accuracy of the machined features and the minimum feature size. On the horizontal plane, a lower accuracy is obtained when scheme 4b is used, as the workpiece material is harder, implying larger tool deflections. Moreover, the presence of burrs is highly undesirable. Most of the limitations of the scheme 4 are to be attributed to the material removal technology used. If microEDM is used for the generation of the patterns on either the metal master or on the steel insert, the minimum feature size would be reduced to approximately 10 µm, and the accuracy of the machined features would be in the order of 1-2 µm, regardless of whether they are concave or convex. Finally, burrs would not form at all. However, if a very high accuracy is not necessary for the functionality and a limited miniaturization is allowed, the solution investigated in this paper, involving the use of micromilling for material removal, allows the integration of a new class of features in the design of plastic microfluidic systems to be produced in large volumes. Conclusions Manufacturing of tools for micro injection molding and hot embossing of microfluidic systems can be realized by several different process sequences. When classified according to the substrate material and whether the pattern is obtained by means of additive or subtractive processes, four main fabrication schemes are identified. Three are based on combinations of photolithography, etching and electrodeposition and the fourth uses thermal or mechanical processes for material removal. The performance of this last fabrication scheme, based on micromilling as material removal technique, was verified in two variants: direct machining in hardened tool steel and machining of a substrate for subsequent electroforming. The overall accuracy as well as the obtainable shapes provided by the investigated method were compared to those provided by the other schemes. When micromilling is used for the patterning of the substrate, tool deflections limit the accuracy on the horizontal plane and burrs formation occurs, which requires further processing and cannot be completely eliminated. On the other hand, real 3D features can be generated, removing the main limitation of the other manufacturing schemes. This investigation shows that, if limited miniaturization is allowed, the proposed process chains involving micromilling permit the integration of a new class of features in plastic microfluidic systems to be produced in large volumes. References [1] R. Trichur, S. Kim, S.H. Lee, Y.A. Abdelaziez, D.E. Starkey, H.B.Halsall, W.R. Heineman & C.H. Ahn, "A

New Plastic CE Chip with Wide Optical Clarity Using Cyclic Olefin Copolymers (COC)", Procedings of Micro Total Analysis Systems 2002, Vol. 1, pp. 560-562, Nara (Japan), November 3-7 (2002)

[2] E.W. Becker, W.Ehrfeld & D. Münchmeyer, Microelectron Eng., Vol. 4., No.35 (1986) [3] J. Elders, H.V. Hansen & M. Elwenspoek, “DEEMO: A New Technology for the Fabrication of

Microstructures”, Proceedings of MEMS’95, pp. 238-243, Amsterdam (1995) [4] M.F. Jensen, J.E. McCormack, B. Helbo, L.H. Christensen, T.R. Christensen, N.J. Mikkelsen, P.T. Tang,

"Rapid prototyping of polymeric microstructures with IR and UV lasers", CIRP seminar on Micro and Nano Technology, Lyngby, November 13-14 (2003)

[5] G. Bissacco, H. N. Hansen, L. De Chiffre, 2004, Improving axial depth of cut accuracy in micromilling, Proc. of the 4th Euspen Int. Conf., Glasgow, Scotland (UK), pp. 386-387.

[6] G. Bissacco 2004 Surface Generation and Optimization in Micromilling, Ph.D. Thesis Technical University of Denmark, IPL246.04.

[7] J. Fugl, 2004, Tooling technologies for micro injection molding, M.Sc. thesis (IPL 152.04), Dept. Manuf. Eng. & Mgn, Technical University of Denmark (in Danish).