Ion track enabled multiple wire microvia interconnects in printed circuit boards

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NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 266 (2008) 1659–1665 www.elsevier.com/locate/nimb

Ion track enabled multiple wire microvia interconnects in printed circuit boards H. Yousef, M. Lindeberg, K. Hjort * Department of Engineering Sciences, The A˚ngstro¨m Laboratory, Uppsala University, Box 534, S-751 Uppsala, Sweden Received 27 September 2007 Available online 22 November 2007

Abstract As the call for higher wiring density in packaging and vertical microvia interconnections (microvias) rapidly evolves, the need for smaller lateral dimensions in printed circuit boards (PCB) microvias must be met. The ion track lithography described in this paper allows for high throughput micromachining of small, deep, vertical microvias in flexible PCB and all-polymer laminates. Ion track lithography makes use of swift heavy ion irradiation to enhance the selectivity and directionality of chemical etching. Within the areas exposed to the ion irradiation, small sub-micron pores (capillaries) are created, one for every ion. If etching is prolonged, the pores become merged. Electrodeposition from a metallic seed layer is used to fill these structures with metal. The lithography masks define either the areas where the ion tracks are developed or where the tracks are metallized. The smallest achievable size of the microvias is only limited by the resolution of the mask; microvias below 10 lm in diameter can also be achieved also in thick polyimide foils. Since each impinging ion forms one track, the foil’s porosity can be controlled by adjusting the irradiation dose, as well as by etching the pores to a suitable size. Depending on the porosity and material, the resultant metallized microvia consists of either individual or interlaced wires (like strands in a bundle wire), or is a solid. As an individual sub-micron wire may have an aspect ratio of several hundreds, this allows for the fabrication of truly vertical microvia structures, allowing ultra-high density microvia batch production. Demonstrator microstructures with highly vertical microvias have been fabricated in foils up to 125 lm thickness. Several components integrated in flexible PCB have been presented by us, e.g. magnetoresistive sensors, thermopile IR-sensors and microwave components like inductor elements. Ó 2007 Elsevier B.V. All rights reserved. PACS: 84.30. r; 81.05.Lg Keywords: Printed circuit board; Polyimide; Foil; Lithography; Ion track; Template

1. Introduction The general trend in the electronics industry is further miniaturization alongside increasing functionality per device and reduced costs. These demands will drive a new generation of low-cost packaging and interconnection technologies for the next decade [1]. Furthermore, several new electronic device application areas such as Intelligent Transport Systems (ITS) for automobiles and wireless sen-

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Corresponding author. Tel.: +46 18 471 3141; fax: +46 18 471 3572. E-mail address: [email protected] (K. Hjort).

0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.11.014

sor networks for e.g. environmental monitoring, are sparking further interest in the development of reliable, low-cost, light weight and multifunctional devices. Flexible printed circuit boards (flex PCBs) are used in a wide range of electronic devices today due to their light weight, bendability, extensive wiring possibilities and lowcost manufacturing techniques [2]. Applications include portable devices, flat panel displays, medical devices, computers, as well as microsystem packaging [3]. Flex PCBs are composed of one or more metal layers with intermediate dielectric layers. In more advanced circuits, the different metal layers are interconnected by plated through-hole vias, allowing for more complex wiring lines and higher

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packing densities. A large number of techniques for fabricating through-hole vias in flex PCBs are in commercial use [4]. Of these, wet etching is the most cost-efficient for producing a large number of vias. Conventional wet etch techniques are isotropic, and therefore via aspect ratios (height-to-width) over 1 are not possible, leading to lower possible packing densities. However, by combining conventional flex PCB manufacturing techniques with ion track technology, our group has presented several techniques for fabricating through-hole vias with aspect ratios up to 4 by wet etching [5–9]. In this paper, a review of these fabrication techniques including recent developments is presented. 2. Ion track lithography in microsystem technology Ion track technology offers unique possibilities for the realization of micrometre to nanometre sized structures in flex PCBs at low-cost and high throughput. Ion track technology is an established technique for fabricating porous plastic membranes. A number of ion track porous membranes are commercially available (e.g. NucleoporeÒ, IsoporeTM) for use in applications such as water purification, blood plasma separation, cell cultivation and air filtration [10]. In research, the porous foils are used as templates for electrodeposition of nanostructures, e.g. [11–16], enabling groups without access to expensive nanostructuring tools to study structures on the nanoscopic scale, e.g. [17–20]. Furthermore, several researchers have used the templates to demonstrate different sensors and/ or analytical tools, e.g. [21–23]. Here, the plastic foils are irradiated with swift heavy ions, inducing nanometre wide tracks of transformed material along the path of the ions referred to as latent ion tracks. The ion tracks exhibit properties that are markedly different from the surrounding bulk material [24–26], and can therefore be selectively etched into high aspect ratio capillaries/pores. In the aforementioned membranes the entire foil surface is irradiated and etched. However, by masking the plastic foils in some way during the steps of irradiation, wet etching or metallization, it is possible to use ion track technology as a fabrication tool for high aspect ratio vertical microstructures. The different masking techniques and resultant structures are reviewed in the following subsections. 2.1. Ion track projection lithography Ion track projection lithography techniques are similar to ion projection lithography or LIGA-like techniques where a pattern is vertically projected from a mask into a radiation sensitive polymer. In LIGA, synchrotron X-ray or ultraviolet light is used to expose a mask. X-ray LIGA allows aspect ratios over 100 with depths up to 10 mm (UV–LIGA only 2 mm). However, the fabrication of projection masks for LIGA and ion projection lithography is often time-consuming and expensive. Furthermore, an

accelerator or synchrotron is required for the lithography step, limiting the use of these techniques [27]. A number of alternative ion beam structuring processes have been presented by others, such as focused ion beams (similar to e-beam lithography), e.g. [28,29], or nano-beam irradiation techniques using an aperture collimator in front of the beam and a moving x–y sample table, e.g. [30]. Here, a promising technique for swift heavy ions is the focused single ion beam technique [31]. However, these techniques are serial and are therefore considered to be slow and costly for large pattern transfer. Ion track projection lithography can be used for patterning larger surface areas. Here, the entire surface area of a sample is irradiated through a mask, as illustrated in Fig. 1. In the first technique (Fig. 1(a)), swift heavy ions with relatively low energies are used at high irradiation fluencies. The ions do not transfer enough energy to create continuous tracks of damaged material, but rather a string of pearl of transformed material. By using high irradiation fluencies, entire volumes can be fully transformed and subsequently etched. In the second technique (Fig. 1(b)), heavier and more energetic ions are used. Each ion transfers enough energy to create a continuous ion track, and lower irradiation fluencies are allowed. Ion track projection lithography has been demonstrated several times, e.g. [32,33], with a recent extension into nanolithography with pattern transfer into amorphous silicon dioxide and titanium dioxide [34–36]. 2.2. Ion track lithography in flex PCB foils To our understanding, only ion track lithography using homogenously irradiated foils can be made affordable for use in flex PCBs. These fabrication techniques have the advantage that the end users, e.g. flex PCB manufacturers, will not need irradiation facilities but may use pre-irradiated (and sometimes pre-etched and metallized) foils from the material providers. Furthermore, homogenously irradiated foils open up for continuous reel-to-reel irradiation which significantly lowers processing costs. We have developed four different processes for fabricating through-hole vias in flex PCB foils, as illustrated in Fig. 2. The processes can be used for fabricating two different types of through-hole via interconnects: conventional vias and multiple wire vias. In conventional vias, the etched pores in a lithographically defined area are merged together to form a via opening for further metallization. In multiple wire vias, separate individual etched pores form a template for electrodepositing bundles of sub-micrometre sized metallic wires. Each wire in the multiple wire vias is analogous to a strand in an electrical cable (stranded wire). Depending on the number and size of the etched pores, the total metal content of the via and, hence, its electrical resistance, can be controlled. The metal content can be adjusted from 0.1% to 100% of the total via volume (the latter corresponding to a conventional via). This allows for both high-resistive vias for use in, e.g. sensor applications,

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Fig. 1. (a) Ion projection lithography using heavy ions with low energy transfer at high irradiation fluencies: (i) Irradiation through a mask. The entire exposed sample volume is transformed. (ii) Etching the transformed volume. (b) Ion projection lithography using heavy ions with high energy transfer and low/medium irradiation fluencies: (i) Irradiation through a mask. Discrete continuous ion tracks are formed. (ii) The ion tracks are etched into individual pores. (iii) With prolonged etching, the pores merge and the entire exposed sample area is etched.

as well as low-resistive vias for use in, e.g. electrical circuitry and RF applications. In the first process (Fig. 2(a)), metal-clad flex PCB foils are used. Both conventional via holes and multiple wire vias are possible with this process, as shown in Fig. 3. Our group has presented inductor coils for microwave circuitry in a copper-clad polyimide foil (Espanex, DuPont) [6,7]. The lowest porosities that are attainable in Espanex foils are limited by the presence of layers of highly imidised polyimide under the metal layers. These highly imidised layers have a bulk etch rate that is up to 3 times lower than in the rest of the foil, resulting in candle-shaped pores (Fig. 3(c)). The lateral resolution of the via apertures is limited due to underetching during the isotropic etch of the thick Cu layers. A further process restriction is that higher ion energies are necessary to penetrate through the whole thickness of the metal layers on the foils. The three process limitations described above can be solved by using bare polyimide foils. As the aspect ratio of the individual wires can be increased, lower via porosities can be achieved. Low porosity vias are useful in sensor applications where higher via resistances lead to higher sensitivity, such as in magnetic field sensors based on magnetoresistance. The low porosity vias are also useful in applications where a thermal gradient is required across

the thickness of the foil such as in vertical thermoelectric structures. Our group has previously presented two processes for fabricating through-hole vias in bare polyimide foils (Kapton HN, DuPont) [8,9]. The two processes are illustrated in Fig. 2(b) and (c). In this paper, we present a third process that has been recently developed in our group (Fig. 2(d)). The motivation behind the development of the technique in Fig. 2(b) was to be able to fabricate high aspect ratio multiple wire vias in a bare Kapton foil. An etched via is shown in Fig. 4. This process was used to fabricate vertical thermopiles consisting of alternating nickel and antimony multiple wire vias in a 75 lm Kapton HN foil [37,38]. This process allows for, today, the highest resolution with via sizes down to 17  17 lm2 and a pitch (centre-to-centre distance) of 20 lm. However, the process is based on a precise control of the ion range into the foil, which puts restrictions on the irradiation parameters (ion energy and or presence of an irradiation mask layer), which may be difficult or costly to implement. The process in Fig. 2(c) was developed as a simpler, more flexible and more cost efficient process to produce multiple wire vias. By developing the process on irradiated foils that are already etched, the end user, e.g. the flex PCB manufacturer, will not need to perform any form of wet

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Fig. 2. (a) Ion track lithography on metal-clad foil: (i) Starting point is homogeneously irradiated foil. (ii) Lithographically defined apertures are wet etched in one of the metal layers. (iii) Pores are etched in the exposed areas. (iv) Top metal layer is removed. (v) Wires are deposited in the pores. The wires merge together when they reach the surface. (vii) Interconnection lithography to couple vias. (b) Ion track lithography on bare foil: (i) Starting point is homogeneously irradiated foil. The foils are irradiated to a specific depth. (ii) Lithographically defined apertures are dry etched into the layer of unirradiated material (iii) Pores are etched throughout the whole foil. Only the pores in the dry etched apertures are open from both sides. (iv) Seed layer deposition on back side of foil. (v) Wires are deposited in pores that are open from both sides. The wires merge and fill the bottom of the dry etched apertures. (vi) Interconnection lithography. (c) Ion track lithography on porous foil: (i) Starting point is homogenously irradiated and wet etched foil. (ii) Seed layer deposition on one side of the foil. A dry photoresist film is laminated on the opposite side of the foil. Apertures are lithographically defined in the resist layer. (v) Wires are deposited in the pores that are in the open areas (apertures) in the dry resist layer. The wires merge and fill the apertures. (vii) Interconnection lithography. (d) Ion track lithography on vertically metallized foil. (i) Starting point is homogenously irradiated, wet etched and metallized foil. (ii) Lithographically defined apertures are dry etched down uncovering the wires. (vii) Interconnection lithography.

Fig. 3. Metallized through-hole vias fabricated by ion track lithography in a metal-clad polyimide foil (Espanex). (a) Multiple wire via consisting of individual wires. (b) Solid via. (c) Candle-shaped top segment of two individual wires.

etching to fabricate the via holes, leading to reductions in process lead times and costs. Resulting vias are shown in Fig. 4. Via sizes down to 33  33 lm2 and pitch of 40 lm

are possible. We believe that it is possible to define and metallize smaller vias by using thinner resist laminates as the masking layer in step (iv) in Fig. 2(c).

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Fig. 4. Through-hole vias fabricated by ion track lithography in a bare polyimide foil (Kapton HN). (a) Top view of an etched via fabricated using the technique in Fig. 2(b). Pores can be seen in the bottom surface of the dry etched aperture. (b) Top view of metallized multiple wire vias fabricated using the technique in Fig. 2(c) after removing the dry photoresist film. The solid metal via ‘‘caps” are a result of the wires merging and filling the dry resist aperture. (c) The individual wires in a via fabricated using the technique in Fig. 2(c). The polyimide foil has been removed by oxygen ashing.

The last process, Fig. 2(d), was developed as a step further in simplifying the process for the end user. Here, the process is developed on irradiated foils that are etched and metallized up to a certain height. This decreases the number of process steps and removes the through-foil wire electrodeposition step, further decreasing process lead times and costs. The remaining steps for the end user will be conventional lithography and dry etching to define the vias. The resulting vias can be used as interconnects for sensorics and electrical circuitry. Furthermore, the metallized foils can be seen as analogous to the anisotropically conductive films (ACF) that are commonly used for flipchip and other packaging applications. A proof-of-princi-

ple of metallized ion track membranes as ACFs was demonstrated in [39] using polycarbonate foils. By developing the process in polyimide as is presented in this paper, the metallized foils can be further structured using lithography, allowing for more complex packaging structures and wiring lines. Furthermore, the polyimide foils can be used as ACFs in applications where higher temperatures are required. Finally, the metallized foils can allow for new application areas such as microwave structures based on ferromagnetic resonance as presented in polycarbonate, e.g. [40,41]. Again, by developing the process in polyimide, it is possible to achieve more complex microwave structures by lithography, as well as higher resolution and packing

Fig. 5. Through-hole vias fabricated by ion track lithography in a vertically metallized polyimide foil. (a) Top view of a metallized via. The aperture was dry etched down to the top of the wires and further metallized. (b) Top segment of individual wires. The wires have two parts: (1) The lower was metallized before dry etching and takes the shape of the pore. (2) The top part was metallized after dry etching and is therefore larger (the dry etch step etches the pores radially). The transition between the two parts can be seen. The polyimide was removed by oxygen ashing. (c) Top view of a whole via. The wires that are not connected to the via can be seen around the via.

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Table 1 Comparison of the four flex PCB ion track lithography techniques

Process starting point No. of steps to attain metallized via Smallest presented metallized via Attainable porosity rangea Main advantage Main disadvantage

Fig. 2(a)

Fig. 2(b)

Fig. 2(c)

Fig. 2(d)

Irradiated metalclad foil 4

Irradiated bare foil

Wet etched foil

Wet etched and metallized foil

4

2

1

39  39 lm2

17  17 lm2

32  32 lm2

65  65 lm2

1–100%

0.1–20%

0.1–20%

0.1–20%

Conventional via hole possible Lowest attainable resolution Lowest porosities not possible

Smallest attainable via holes

No via etching

Precise control of ion range into foil necessary

Lower resolution

No via etching or metallization Cannot be used in applications where electromagnetic compatibility is necessary (e.g. RF)

a Lowest porosity calculated for 100 wires in a 40  40 lm via (100 wires per via lead to an acceptable statistical variation in the number of wires per via).

densities. The lowest attainable via dimensions using this process are dependant on the resolution of the lithographic step. Vias with a side length of 65 lm and a pitch of 280 lm are presented in Fig. 5. A comparison of the four processes in Fig. 2 can be found in Table 1. 3. Engineering ion track enabled flex PCBs In engineering, when choosing a fabrication technology, cost and availability must always be considered. The cost of the fabrication techniques in Section 2.2 are analysed below. The cost of irradiation depends on the ion track density and if continuous reel-to-reel irradiation is possible. Commercial reel-to-reel irradiation of thin foils can produce in the order of 1012 tracks per second at a cost of €250 per hour, which equals an area of 1 m2 with a track density of 108 cm 2 (1 track per lm2) per second. For thicker foils, we have experienced irradiation currents that produce typically 109 tracks per second at a cost of €1000 per hour. The cost of ion track etching depends on which etchant is used and on the capital costs of the etching equipment. In a reel-to-reel continuous etching machine, areas of up to 5 m2 can be etched at the same time. In the processes presented in this paper, the pores are etched to a diameter of 100–300 nm within 2 h, corresponding to an etch throughput that is in the order of 50 m2 per day and machine. If frames of flex PCB foils are to be etched, the work will be more labour intensive, and less than 0.5 m2 per etch batch can be handled. The flex PCB manufacturing industry is highly cost sensitive. The motivation for adopting a new fabrication technique will depend on whether other more mature flex PCB technologies can be used instead. However, in the case of moderately high ion track densities (up to 108 ions/cm2) and continuous reel-to-reel processing, the additional costs to the process due to irradiation and etching will be minute relative to the overall costs. Here, cost concerns will lie on

adapting the remaining process steps to the established flex PCB processing line. For example, the electrodeposition step differs from conventional flex PCB through-hole metallization, and we are today not able to achieve electrodeposition with sufficient uniformity at deposition rates higher than 1 lm/min. To be a viable technique in the flex PCB industry, this step must be further optimized to attain uniform deposition over larger panel areas at higher deposition rates. On the other hand, this development process may prove to be cost-efficient in the long run as the cost of steps such as laser micromachining or dry etching is omitted, and much higher packing densities are achievable. 4. Future outlook Will ion track lithography be used in high resolution flex PCB technology? To our understanding, cost will not be a key factor in answering this question. The major considerations will be if any specific circuitry, such as integrated sensorics, can be realized by the processes, or if the circuitry must be done in thick polymer foils for compatibility issues in, e.g. microwave applications. As long as the increased demand of higher resolutions may be solved by using thinner dielectric foils, we see no immediate need for ion track enabled through-hole vias, and we therefore believe that mature technologies will prevail even if they may be somewhat more costly. Furthermore, only a few medium sized ion accelerators are available for polymer foil irradiation today. Without a secured second source, most system providers will not commit to a technology. Hence, for the time being, we find that ion track lithography is only interesting in niche applications such as low-cost sensors. 5. Conclusions Several different possibilities for fabricating high aspect ratio through-hole vias in flex PCBs by combining ion track

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technology with conventional PCB manufacturing techniques are presented. The presented processes allow for vias with aspect ratios up to 4, and a total metal content ranging from 0.1% to 100%. Each of the processes has its benefits and drawbacks for different applications when it comes to attainable dimensions and porosities, as well as process steps and costs. In this way, a wide range of fabrication needs can be fulfilled by one or more of the presented processes. To become a viable technology for the flex PCB industry, the biggest challenge today lies in developing the process steps, in particular the through-hole electrodeposition, towards higher throughputs and larger panel areas. Nonetheless, with the scarcity of irradiation facilities for foils that are thicker than 25 lm, we find that for the moment, the processes are only interesting for niche products. Acknowledgements Part of the work was financed by the European Commission through the Human Potential Program Network EuNITT, Contract No. HPRN-CT-2000-00047, the Swedish Agency for Innovation Systems, VINNOVA, through the Competence Centre SUMMIT and the Forska&Va¨x Programme, and the Swedish Foundation for Strategic Research through the Research School AME. References [1] The International Electronics Manufacturing Initiative (iNEMI), 2007 iNEMI Research Priorities, , 2007. [2] J. Fjelstad, Flexible Circuit Technology, third ed., BR Publishing, Oregon, 2007. [3] D.J. McKenney, D.K. Numakura, CircuiTree 13 (2000) 86. [4] C.F. Coombs Jr., Coombs’ Printed Circuits Handbook, fifth ed., McGraw-Hill, New York, 2001. [5] M. Lindeberg, K. Hjort, Sensors Actuators A 105 (2003) 150. [6] M. Lindeberg, K. Hjort, Microsyst. Technol. 10 (2004) 608. ¨ jefors, A. Rydberg, K. Hjort, in: [7] M. Lindeberg, H. Yousef, E. O Materials Research Society Symposium Proceedings, Materials, Integration and Packaging Issues for High-Frequency Devices II, Boston, USA, 29 Nov–1 Dec, Vol. 833, 2005, p. 255. [8] H. Yousef, K. Hjort, M. Lindeberg, J. Micromech. Microeng. 17 (2007) 700. [9] H. Yousef, K. Hjort, M. Lindeberg, J. Micromech. Microeng. 18 (2008) 017001. [10] P. Apel, Radiat. Meas. 34 (2001) 559. [11] C.R. Martin, Science 266 (1994) 1961. [12] J. Vetter, R. Spohr, Nucl. Instr. and Meth. B 79 (1993) 691. [13] C. Schonenberger, B.M.I. van der Zande, L.G.J. Fokkink, M. Henny, C. Schmid, M. Kruger, A. Bachtold, R. Huber, H. Birk, U. Staufer, J. Phys. Chem. B 101 (1997) 5497.

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